A batting helmet is a hard plastic hat worn over a player's normal baseball cap. While the name implies that it is worn exclusively by batters, it is actually worn by offensive players in every stage of their appearance on the field: while on deck, in the batter's box, and running the bases.
Batting helmets developed surprisingly late in the history of baseball. The danger of being hit by a pitched baseball were apparent from very early on, as shown by the early development of the catchers' "tools of ignorance". Both catcher's masks and chest protectors were in common use by the 1890s, and shin guards were adopted in the 1900s. Batting helmets were not required for another half century. Even the tragic death of Ray Chapman, who was fatally beaned by Carl Mays in 1920, did not lead to the adoption of protective gear.
The first known use of batting helmets in the majors was by the 1941 Brooklyn Dodgers. Two of the Dodgers, Joe Medwick and Pee Wee Reese, had suffered severe beanings, so General Manager Larry MacPhail made the entire team wear protective helmets. Those helmets were based on jockeys' helmets, and were much like a normal baseball cap with a hard liner.
In 1971, Major League Baseball made helmets mandatory, though some veterans continued to wear cloth caps with liners under a grandfather clause. They included Norm Cash, Bob Montgomery, and Tony Taylor.
Starting in 1983, players were required to wear helmets with flaps extending down from the crown to cover the ear of the batter that was closest to the pitcher. Players who had previously worn flapless helmets were allowed to continue doing so. Tim Raines Sr. was the last active player to wear a flapless helmet. This innovation began in the 1960s as Earl Battey and Tony Oliva improvised flaps. The first man to wear a helmet with a flap that was molded in as part of the factory construction was Tony Gonzalez, in 1964.
In 2013, Major League Baseball introduced a new model of helmet manufactured by Rawlings, the S100 Pro Comp, which has been shown in tests to be able to withstand the impact of a 100 mph fastball (previous models were only tested to 68 mph). It is constructed of aerospace-grade carbon fiber composite and was made mandatory throughout the major leagues, as a provision of the 2011 Collective Bargaining Agreement.
Players are allowed to wear helmets that are more protective than the standard design. A handful of batters have worn helmets with cheek protectors, often after having suffered face injuries. Gary Roenicke and Ellis Valentine are two examples. Safety advocates have encouraged youth leagues to adopt helmets with a full facemask, much like those used by cricket batsmen, but response has been tepid.