The Boston Amateur Baseballers' Union (BAmBUu, "with an extra U for Up Yours!") was officially established in 2002, but the league's teams have been playing in an unofficial Sunday tradition that stretches as far back as anyone can remember.

BAmBUu is competitive but not cut-throat, and good fun is had by all. Players in the league range in age from 18 to 40-something, and hail from all over Boston and the Boston Metropolitan area (even one team from New Hampshire). Regular baseball rules apply, with a few exceptions which league officials believe make for a more accessible and safer version of the National Pastime.

The League currently consists of eight teams: the Bay State Ruck, the Beantown Basers, the Boston Redsleeves, the Fenway Hustlers, the Dorchester Dirtbagz, the New Hampshire 603s, the Quincy Maggots, and the South End Saints. Fans often refer to the Redsleeves as "the 'Sleeves" and the Dorchester Dirtbagz as the "D-Bagz". No nicknames have officially been assigned to the two new teams, the Maggots and the Saints, though there has already been references to the "Mags", and the, well, Saints.