Teams make trades for specific reasons. Sometimes, they make a deal just to fill a void, to replace an injured player, to add a piece for a playoff run. Sometimes they deal to get more payroll flexibility.
But the most fascinating trades are the ones that alter the team's makeup. One general manager tells me that this is the real purpose of trades; that you can acquire players in a variety of ways but if you want to fundamentally change the makeup of your team, that usually comes via trade.
With that in mind, we look at 30 trades for 30 teams. We are not necessarily looking for the team's BEST trade, though some do fit that description. We are looking instead for the trade that the team needs to work, the trade that will play a big part in getting the team where it wants to go.
AMERICAN LEAGUE EAST
Dec. 11, 2017: Yankees trade for Giancarlo Stanton (New York trades Starlin Castro and two Minor Leaguers to Miami for Stanton)
This one is obvious; many expected the Yankees to wait it out and make a big play in the Bryce Harper/Manny Machado free agency party next year. Instead, they dealt for the National League Most Valuable Player and home run champion, who is signed through 2027. The move will unquestionably have a significant financial impact for years to come -- it may affect the Yankees' plan for when players like Aaron Judge and Luis Severino are eligible to become free agents. But if Stanton stays healthy and hits like he did in 2017, it will be a team-altering move, which is exactly why GM Brian Cashman did it.

Dec. 6, 2016: Red Sox trade three Minor Leaguers and major prospect Yoan Moncada for Chris Sale.
The plan here is for the Red Sox to put together perhaps the best ever lefty-pitching combo -- Sale and David Price. This didn't work as well as expected in 2017 when Price struggled with injuries and the media, and Sale had some late-season difficulties that carried into the postseason. But Red Sox president Dave Dombrowski came to Boston to win now, and for that to happen, the Sale-Price duo needs to dominate.
Jan. 19, 2018: Blue Jays trade Dominic Leone and a Minor Leaguer for Randal Grichuk.
One move the Blue Jays did NOT make -- not dealing Josh Donaldson -- suggests they want to compete in the American League East. Grichuk could be a key player in Toronto's efforts to surprise. He has massive power -- he has hit 24 and 22 home runs the last two seasons even though he did not get 500 plate appearances in either -- and Toronto is a home run hitter's paradise. The question with Grichuk is contact; will he connect often enough to overcome his high-strikeout, low-walk history? 
Dec. 20, 2017: Rays trade Evan Longoria to San Francisco for Denard Span and three players.
The idea here is to talk about trades that remake a team's identity, and none does this more than the Rays dealing Longoria. He was the face of the franchise for a decade, a guy the Rays had under team control through 2023. The players they got back in the deal may or may not pan out -- scouts like third-base prospect Christian Arroyo, though he has already been sent back to the Minors -- but the decision to trade Longoria, free up payroll and reinvent baseball in Tampa Bay will be the one that defines Rays baseball for years to come.
Orioles did not trade Manny Machado
The Orioles had a difficult decision to make this offseason -- trade off Machado and begin the heavy lifting of rebuilding the team or keep him for likely one more year and try to make one final unlikely run. The Orioles decided to keep him. If you want to go back in time, you can point out that they traded Erik Bedard for a slew of players, particularly Adam Jones, who played a major role in turning the Orioles into contenders for a time. But this is a team in need of a makeover, and it seems the Orioles keep pushing off the inevitable.

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        AMERICAN LEAGUE CENTRAL
        July 31, 2016: Indians trade four Minor Leaguers to New York for Andrew Miller
        Several teams -- the Cubs and Indians in particular -- have built their teams with talented young everyday players and by acquiring veteran pitching. Four of the five likely Cleveland starters were picked up by trade, including ace Corey Kluber. But it was the acquisition of superweapon Miller that set up the Tribe for their stunning run to Game 7 of the 2016 World Series. And Cleveland's hopes in 2018 against loaded superteams like Houston and New York will build around its belief that it has the best rotation and a dominant bullpen anchored by Miller.
        Dec. 6, 2016: White Sox trade Sale to Boston for Moncada, Michael Kopech and two other Minor Leaguers.
        This was also the most important trade for Boston, but for very different reasons. The Red Sox are in it to win it now. The White Sox tore the team down and by acquiring mega-prospects Moncada (last year's No. 2 prospect) and Kopech (No. 10 per MLB Pipeline), they began the rebuilding process in style. The White Sox also have picked up other top prospects in trades (including former No. 3 prospect Lucas Giolito and current No. 4 prospect Eloy Jimenez), but Moncada and Kopech are both expected to be factors in 2018 and if they turn out, the White Sox will feel pretty good about the future.
        Jan. 4, 2018: Royals trade Joakim Soria and Scott Alexander in a three-way deal that brought Travis Oaks and Erick Mejia.
        To follow the mission of this piece, the big trade for the Royals involved not dealing their would-be free agents last year -- Lorenzo Cain, Eric Hosmer, Mike Moustakas -- and instead taking one more run at the playoffs. That decision will define the Royals the next few years. But by dealing Soria they did free up some payroll, and by moving Alexander they got a couple of younger players who could be part of the future. It will be interesting to see if the Royals consider trading their two most valuable chips right now, catcher Salvador Perez and pitcher Danny Duffy.
        Feb. 17, 2018: Twins trade prospect Jermaine Palacios to Tampa Bay for Jake Odorizzi.
        The Twins are coming off a season where they sneaked into the playoffs behind a surprisingly robust offense (fourth in the AL in runs) and an excellent defense. With some exciting young players, the time is now in Minnesota, but that rotation simply wasn't going to cut it, especially after Ervin Santana went down with an injury. Trading for Odorizzi to join promising Jose Berrios and free agent Lance Lynn, the Twins have unquestionably revamped their rotation. They need the best version of Odorizzi for it to work.

Aug. 31, 2017: Tigers trade Justin Verlander to Houston for four Minor Leaguers, including Franklin Perez, Jake Rogers and Daz Cameron, who rank in the system's Top 10.
For years, this was exactly what the Tigers seemed unwilling to do -- break things up and start anew. It undoubtedly hurt to trade the franchise centerpiece Verlander, whom you would expect to go to the Hall of Fame someday as a Detroit Tiger. But the Tigers made the decision to infuse the system with youth, open up some payroll possibilities and rebuild for the future. They need to be right on the prospects, particularly Perez, who is only 20 but has a chance to be in the rotation sooner rather than later.
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AMERICAN LEAGUE WEST
Nov. 12, 2015: Angels trade Erick Aybar, Sean Newcomb and a Minor Leaguer for Andrelton Simmons.
Sometimes you don't fully appreciate the possibilities of a trade when you make it. Simmons was known to be a defensive genius at shortstop in Atlanta but the lingering question was whether he would hit enough to be an impact player. Last year he took a quantum leap forward as a hitter -- setting career highs in almost everything -- and was a legitimate MVP candidate. He is now a centerpiece along with Michael Trout for the Angels' efforts to win right now.
Dec. 7, 2017: Mariners trade three Minor Leaguers to Miami for Dee Gordon
The Mariners offense was middle-of-the-pack last year in basically every way -- they were sixth or lower in every single offensive category except hit-by-pitch, which is hardly a dynamic thing to be good at. General manager Jerry Dipoto has been aggressive about making the team more athletic, and so he traded off prospects -- including highly-touted pitcher Nick Neidert -- to get one of the game's fastest players. Gordon has led baseball in stolen bases three of the last four years, and he scored 114 runs last season. The Mariners desperately hope he will add dynamism to a lineup that did not have a player score more than 91 runs last year.
July 31, 2015: Rangers trade five players to Philadelphia for Cole Hamels.
It has been more than two years since the Rangers made this deal, but it is still the one that defines their future. Hamels remains the Rangers' No. 1 starter, at least in name, as the team shuffles its way forward. With veterans like Adrian Beltre, Elvis Andrus and Shin-Soo Choo, among others, expected to play every day, the Rangers hope to contend in what looks to be the brutally competitive American League West. For that to possibly happen, they will need Hamels to return to 2015 form.
Aug. 31, 2017: Astros trade four Minor League players for Verlander
The Astros appear to be getting close to building the perfect roster. They won 101 games last season and won the World Series with a very young roster … and then this offseason they traded for former No. 1 overall pick and All-Star Gerrit Cole. But the trade that pushed the Astros over the top was acquiring Verlander, who was named AL Championship Series MVP. The Astros have Verlander for the next two seasons, and if he's good, the Astros have a great shot of going back-to-back.

July 31, 2017: Athletics trade Sonny Gray to New York for Dustin Fowler and two Minor Leaguers.
There are really two trades of equal importance for the A's. The first was the controversial Donaldson trade to the Blue Jays. Few people around baseball liked that deal for Oakland, but the A's have Kendall Graveman slotted in as their No. 1 starter and shortstop Franklin Barreto is ranked as a Top 100 prospect, though his stock has slipped in the minds of some scouts. The second: the A's, in an effort to remake their team, dealt Gray and brought back Fowler, a tremendous talent who suffered a grisly knee injury. Fowler is slotted to play center field every day, and the A's need him to be healthy and a force as they work toward the future. Pitcher James Kaprielian, who also came over in that deal, is a hard-throwing righty who has hit 99 mph with the fastball, but he is coming off Tommy John surgery.
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NATIONAL LEAGUE EAST
July 31, 2015: Phillies trade Hamels for Jerad Eickhoff, Nick Williams, Jorge Alfaro and two Minor Leaguers.
The Hamels trade is a perfect reflection of what a deal can do to change the makeup of two teams. The Rangers were a real contender, and Hamels helped them get into the postseason and be a factor. The Phillies were a shell of the successful team that dominated for four or five years, and the deal helped them rebuild. Alfaro will be their starting catcher and he is a promising offensive player. When he returns from a right lat injury, Eickhoff will be in the middle of the rotation that now looks much better with the free-agent signing of Jacob Arrieta. And Williams looks like he can really hit. For the Phillies to take the next step, these three must lead.
Dec. 17, 2012: Mets trade R.A. Dickey to Blue Jays for Travis d'Arnaud and Noah Syndergaard.
This was a coup. Dickey had just come off a Cy Young season at age 37, a wonderful season that frankly did not match the rest of his career. The Mets took advantage of the moment by dealing Dickey to Toronto for d'Arnaud, who was widely viewed as one of the two or three best catching prospects in baseball. In addition, they got a still raw but impossibly hard-throwing Class A prospect named Syndergaard, who has developed into one of the most dominant pitchers in the game. 
Dec. 9, 2015: Braves trade Shelby Miller to Arizona for Aaron Blair, Dansby Swanson and Ender Inciarte.
This deal shocked the baseball world, in large part because Arizona had drafted Swanson No. 1 overall just six months earlier. Swanson is still an open question -- his talent is apparent but he hit just .232 with almost no power in his first full season in the big leagues at 23. Blair has gone backward and is trying to resuscitate his career (he came to camp 40 pounds lighter). But Inciarte has been a revelation; he's a defensive dynamo, superb on the basepaths and last year hit .300. Both he and Swanson are under club control until 2022.

July 16, 2017: Nationals trade Blake Treinen and two Minor Leaguers for Sean Doolittle and Ryan Madson.
We went back and forth on this one. The Nationals are obviously relying heavily on the deal with the White Sox that brought over outfielder Adam Eaton (the Nats traded mega-prospect Giolito in that one). Eaton is a key to the Nationals' contention run this year, but let's go with the Doolittle-Madson deal if, for no other reason, because for the first time in a long while they're not freaking out about the bullpen in Washington. 
Jan. 25, 2018: Marlins trade Christian Yelich to Milwaukee for Lewis Brinson and three Minor Leaguers.
Everyone knows that Miami is tearing things up entirely -- in the offseason, they traded Stanton, Gordon, Marcell Ozuna and Yelich in a complete payroll purge. The trade that best seems to reflect what the Marlins hope will come out of this is the Yelich deal; they got Brinson, a hugely gifted player who looks to be the Miami's starting centerfielder right away. Another outfielder, Monte Harrison, is the team's No. 2 prospect and in everybody's Top 100. The Marlins need to cash in on this one.
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NATIONAL LEAGUE CENTRAL
July 13, 2017: Cubs trade top prospect Eloy Jimenez and three Minor Leaguers to the White Sox for Jose Quintana.
This was a huge trade for several reasons. For one thing, it brought the Cubs a solid third or fourth starter that is under team control for the next three seasons. Quintana with Jonathan Lester, Kyle Hendricks, Yu Darvish and Tyler Chatwood make up one of the best rotations in the NL. But, Theo Epstein says, perhaps even more crucial is that the Cubs probably could not have signed Darvish had they not picked up Quintana -- getting a reasonably priced pitcher who is under team control is what freed up Chicago to add a pitcher of the caliber of Darvish without having to deal from their core group of players.
Jan. 19, 2017: Reds trade Dan Straily to the Marlins for Luis Castillo.
Sometimes what general managers call "small-print" moves turn out to be team-altering. The Reds picked up Straily on waivers, he had success, and the Reds quickly dealt him to Miami for Castillo, who hits triple digits with his fastball and showed tremendous promise in his 15 starts in 2018. The Reds offense is interesting, with the great Joey Votto in the middle and power everywhere, so if Castillo can become an ace, some interesting and surprising things can happen.
Dec. 14, 2017: Cardinals trade four players to Miami for Ozuna.
In many ways, the Cardinals have been searching for an identity ever since Jose Pujols left for California. They have had great success since his departure -- reaching the postseason four straight years and winning one pennant -- but the team has had five different WAR leaders in those six years. Ozuna is coming off a massive offensive season with 37 homers, 124 RBIs and a .548 slugging percentage -- the last Cardinals player to put up these sorts of numbers was Pujols himself. The Cardinals have missed the postseason the last two years; Ozuna's success will help determine if they can turn things around this year.
Jan. 13, 2018: Pirates trade Cole to Houston for Joe Musgrove, Colin Moran, Michael Feliz and Jason Martin.
The Pirates made two massive trades this offseason in an effort to reshape the roster. The one that got the most attention, for obvious reasons, was their dealing Pirates legend Andrew McCutchen to the Giants. But you could argue that the Cole trade was even more impactful. Musgrove looks to be in the middle of the Pirates rotation, Moran is an interesting talent who is set to start at third base and Feliz is a strikeout pitcher the Pirates hope will excel in the bullpen. The Pirates' future relies on them being right in this deal.

July 30, 2015: Brewers trade Carlos Gomez and Mike Fiers to Houston for Domingo Santana, Josh Hader, Brett Phillips and Adrian Houser.
You could argue that this offseason deal for Yelich is the key to a season where the Brewers feel like they can contend, but let's go back to this classic from 2015. Doug Melvin wanted to retool the Brewers and he brought in four young players, all who could play a significant role in the team's future. Santana smashed 30 home runs last year and looks like a star in the making. Hader was overpowering in his rookie year in the bullpen and he might be the Brewers' eighth-inning guy. Phillips was awfully good after getting called up late last season and he is a defensive wizard in the outfield. And even Houser, who is coming off Tommy John surgery, has shown great promise since returning. This was actually Melvin's second choice -- he had worked out a deal sending Gomez to the Mets for Zack Wheeler -- but this is the one that marks the promising Milwaukee future.
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NATIONAL LEAGUE WEST
Dec. 11, 2014: Dodgers trade Gordon and Dan Haren to Miami for Austin Barnes and Enrique Hernandez.
Dodgers general manager Farhan Zaidi picks out this deal specifically because it so represents the extraordinary flexibility of this Dodgers roster. Barnes is the Dodgers' starting catcher -- and one of the best in the NL -- but he can also play second base or third in a pinch. Hernandez is the backup at all three outfield positions and he can play shortstop when they need him there. 
Dec. 5, 2014: D-backs trade Didi Gregorius in a three-way deal to Yankees for Robbie Ray.
Went back and forth on this one. On the one hand, the offseason acquisition of Tampa Bay's Steven Souza Jr. was huge -- he hit 30 home runs in Tampa Bay last year, he is moving into a much better hitter's park (even with the humidor being added) and the D-backs desperately need him to fill the power void left behind when J.D. Martinez went to Boston. But on the other, Ray's electrifying transformation into a dominant pitcher last year was one of the big reasons -- perhaps the biggest reason -- Arizona surprised and made the postseason.

June 4, 2016: Padres trade James Shields to the White Sox for Minor Leaguer Fernando Tatis Jr.
When you are on the rebuilding road, you desperately need some surprising things to go right. When the Padres traded Shields in the middle of the 2016 season, general manager A.J. Preller was a man still in search of an identity. What were the Padres? They had one foot in the "contending" bucket and another in the "rebuilding" bucket. They traded Shields and took a flier on a supremely talented 17-year-old, Fernando Tatis Jr., son of the fine Major Leaguer. Now, Tatis is one of the best prospects in the game, the Padres have what MLB Pipeline considers the best and deepest Minor League system in baseball, and the future is looking bright in San Diego.
Jan. 28, 2016: Rockies trade Corey Dickerson to Tampa Bay for German Marquez and Jake McGee
You look at the makeup of the Rockies, and you see a team that is largely built through high Draft picks -- Trevor Story and Jon Gray were first-round picks, Nolan Arenado and Charlie Blackmon were second-round picks, and so on. They have made some successful deals here and there -- picking up All-Star second baseman DJ LeMahieu from the Cubs -- but mostly it has been working from within. The Dickerson deal, though, has helped change the pitching outlook in Colorado. Marquez is the Rockies' No. 3 starter, though he showed enough as a 22-year-old rookie last year to hope for more. McGee is a veteran, but he did help lock down the bullpen last year and should play a significant role as the Rockies try to get back to the postseason.
Dec. 20, 2017: Giants trade Span, Arroyo and Minor Leaguers to Tampa Bay for Longoria.
The Giants are coming off their worst season in more than 30 years, so they hit the market hard, trying their best but failing to reel in Stanton and Ohtani but then trading for two of baseball's most iconic players. They traded for Pittsburgh's McCutchen. But, even more telling, they dealt for Tampa Bay's Longoria, who still has five years and a team option left on his deal. The Giants have long been about winning now, and this season it's more true than ever -- the Giants are hoping for Longoria and McCutchen to find their past form as superstars.