There are four trolleybus routes in the Boston, Massachusetts area, all run by the Massachusetts Bay Transportation Authority (MBTA) in the Harvard Square area, and all former streetcar lines (the last four not connected to the Tremont Street Subway to survive). The MBTA and its predecessors once ran a large system of trackless trolleys that covered much of the lines north of Boston as well as separate groups in Dorchester and Jamaica Plain. The four current routes were created after much of the system was converted to diesel and exist, in part, because of the necessity to limit the number of diesel buses in the Harvard Tunnel. They are all stored overnight in the North Cambridge Carhouse, the northern terminus of the 77A. Trackless trolleys do not run on Saturday evenings and Sundays; instead diesel buses cover the routes.
In 2004, the MBTA began receiving a fleet of new trackless trolleys from Neoplan, to replace older Flyer trolleybuses dating to the 1970s. The newer vehicles meet Americans with Disabilities Act standards (see MBTA accessibility).
Additionally, the Silver Line Phase II uses dual-mode buses and until early 2006 also used some of the new Neoplan trackless trolleys pending the entry of the entire dual-mode fleet into service. As of July 2006, the Neoplan trackless trolleys have now all been transferred to the following routes, and most of the Flyer trolleybuses have been retired, with no more than a dozen being kept as spares.