The official rules of Major League Baseball require that all players on a team wear matching uniforms, although this rule was not in force in the early days. Originally, teams were primarily distinguished by the colors of their stockings and the success of the Cincinnati Red Stockings popularized the adoption of sock color as the explicit identity of the club. The 1876 Chicago White Stockings actually wore caps of different colors. In 1882, the National League assigned stocking colors to the member clubs: red for Boston, white for Chicago, grey for Buffalo, blue for Worcester, gold for Detroit, green for Troy, and so on. That year, the league also assigned jersey and cap colors, but by player position rather than by club.

Shoulder CordsTraditionally, when playing at home, teams wore uniforms that were mostly white with trim in team colors and when playing away, they wore uniforms that were mostly gray with trim in team colors. Aside from the obvious need to distinguish one team from the other, conventional wisdom held that it was more difficult to properly launder uniforms while on a road trip, thus the "road grays" helped to hide accumulated soil. This convention continued well after its original premise was nullified by the issuance of multiple uniforms and the growth of the laundromat industry. Starting in the 1970s, with the advent of synthetic fabrics, teams began using more color in their uniforms, notably the Kansas City Athletics in 1963, the San Diego Padres unusual brown and yellow scheme beginning in 1969, and the Houston Astros' rainbow stripes in the mid-1970s. In the late 1970s, the Pittsburgh Pirates began a trend of multiple combinations of differently colored jerseys and trousers and caps (with the options of black, yellow, and white with pin stripes). At one point in the 1970s, the Cleveland Indians had an all-red uniform.

Starting in the 1990s, MLB clubs began heavily marketing licensed goods, such as caps and uniform jerseys to the public and this has resulted in a wide array of uniforms for each team. Now, some teams have not only a basic home uniform and away uniform, but also special "Sunday game" uniforms and uniforms that are worn only during batting practice and uniforms worn on singular events. From time to time, individual MLB teams have held "Turn Back the Clock Day", regularly-scheduled games in which teams donned uniforms in styles their predecessors wore generations earlier (sometimes called "throwback" uniforms), or other antique-style uniforms such as those of Negro League clubs. The Los Angeles Dodgers occasionally use the livery of their original identity as the Brooklyn Dodgers, on special anniversaries or occasions, for example such as in honor of the retirement of Jackie Robinson's uniform number 42 throughout professional baseball. In addition, in 1999, MLB staged "Turn Ahead the Clock Day," in which teams wore futuristic, somewhat strange-looking uniforms, including futuristic or science fiction references, such as the New York Mets being referred to as the "Mercury Mets."

The result is that it is now often difficult to say which uniform is a team's "official" one. For example, the Cincinnati Reds used to wear a variety of caps: all red, red crown and black bill, black crown and red bill, and all black, until 2007, when only the all-red and red crown/black bill was brought back. In contrast from the pre-1990s era, in which there usually was just one home uniform and one road uniform (with certain exceptions, such as Oakland and Pittsburgh's complex combinations), today choices of what combination of uniform elements are worn are now sometimes left up to players. In some cases, aspects of the uniform that are considered official are now rarely worn, such as the New York Mets' all-blue home cap, which is rarely seen on the field today in favor of an "alternate" black-and-blue cap. Through 2007, The New York Yankees, Detroit Tigers, St. Louis Cardinals, San Francisco Giants, Los Angeles Dodgers, and Chicago Cubs were the only teams that did't wear alternate uniforms.

The Cubs, Dodgers and Giants had worn alternate uniforms in the past, but as of 2007 did not have one. In 2008, the Chicago Cubs reintroduced their alternate, all Blue uniform, while the Tampa Bay Rays did not include an alternate uniform in their redesign, becoming the only expansion team without an alternate uniform. Typically, home uniforms feature the team's logo, insignia, or nickname, while away uniforms feature the name of the team's home city or state. Currently, the Tampa Bay Rays, Baltimore Orioles, Los Angeles Angels of Anaheim, Philadelphia Phillies, St Louis Cardinals, and Milwaukee Brewers are the only exceptions to this rule. These teams feature the club nickname on both the home and away uniforms.