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.

Traditionally, 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 Oakland Athletics in the early 1970s and the Houston Astros 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).

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. On several occasions, the MLB has instituted nostalgia events, during which teams wore uniforms from the past.

The result is that it is now often difficult to say which uniform is a team's "official" one. For example, the Cincinnati Reds now wear a variety of caps: all red, red crown and black bill, black crown and red bill, and all black. 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.

The official rules state that:

All players on a team must wear identical uniforms during a single game.

Numbers: All players must wear their uniform numbers on the back of the uniform

Undershirt: If the undershirt is exposed then all the players on the team must wear matching ones. Numbers or other devices may be worn on the sleeve of the undershirt (for example, if it is worn with a sleeveless jersey), except that pitchers may not have such devices on their undershirt sleeves.

The league office might require that each team have a single uniform for all games or requires that each team have a single, white home uniform and a single, non-white away uniform. With the elimination of the separate American League and National League administrations, it is unknown what the effectiveness of this rule now is.

Sleeve length: The rules allow for minor variation in sleeve length, but they must be "approximately the same length" and the sleeves may not be "ragged, frayed or slit."

No attachments: Tape or other attachments of non-matching color may not be used on uniforms.

No images of baseballs: No "pattern that imitates or suggests the shape of a baseball" may be used on uniforms. Notably, in apparent violation of this rule, the Milwaukee Brewers and Philadelphia Phillies for many years had logos that incorporated the image of a baseball. The Brewers logo has made a comeback in 2006 on the hats of the Brewers' Sunday home uniforms. Also, many teams such as the Giants use sleeve logos that clearly depict a baseball, so it may be that the rule is not enforced. (The purpose of this rule is to prevent one team from deceiving the other. The National Football League has a similar rule, which states that no pattern that imitates or suggests the shape of a football).

No glass buttons or polished metal

No commercial advertisements on uniforms. This rule is in variance with professional sports, especially outside the United States (notably soccer), in which it is customary for uniforms to prominently display the logo of a sponsoring company.

Names: "A league may provide that the uniforms of its member teams include the names of its players on their backs. Any name other than the last name of the player must be approved by the League President. If adopted, all uniforms for a team must have the names of its players." Again, with the elimination of separate administrations for the American and National leagues, it is unknown what the provenance of this rule is. (Currently, Ichiro Suzuki of the Seattle Mariners is the only player to have his given name rather than his family name displayed on the back his uniform. Vida Blue also used his first name on the back of his uniform when he played for the San Francisco Giants in the mid-1980s). The Chicago Cubs, New York Yankees, Boston Red Sox, Los Angeles Dodgers and San Francisco Giants do not display their players' names on their home uniforms (The Yankees and Dodgers also don't display them on their road uniforms), although the Cubs and Dodgers will return names to the back of all their uniforms in 2007. The New York Mets used alternate home uniforms without last names for several years in the late 1990s and early 2000s.