The MBTA bus system is the nation's sixth largest by ridership and comprises over 150 routes across the Greater Boston area.
The area served by the MBTA's bus operations is somewhat larger than its subway and light rail service area, but is significantly smaller than that served by the MBTA's commuter rail operation.
At least eight other regional transit authorities also provide bus services within that larger area, these being the Rhode Island Public Transit Authority, Brockton Area Transit Authority, Cape Ann Transportation Authority, Greater Attleboro Taunton Regional Transit Authority, Lowell Regional Transit Authority, Merrimack Valley Regional Transit Authority, Montachusett Regional Transit Authority, and Worcester Regional Transit Authority.
All of these authorities have their own fare structures and some subcontract operation to private bus companies.
In many cases, their buses serve as feeders to the MBTA commuter rail.

Within MBTA's bus service area, transfers from the subway are free if using a CharlieCard (for local buses); transfers to the subway require paying the difference between bus and the higher subway fare (for local buses; if not using a CharlieCard, full subway fare must be paid in addition to full bus fare).
Bus-to-bus transfers (for local buses) are free unless paying cash.
Many of the outlying routes run express along major highways to downtown.
The buses are colored yellow on maps and in station decor.

The Silver Line is the MBTA's first service designated as bus rapid transit (BRT), even though it lacks many of the characteristics of bus rapid transit.
The first segment began operations in 2002, replacing the 49 bus, which in turn replaced the Washington Street Elevated section of the Orange Line.
A full subway fare was charged, with free transfers to the subways downtown until January 1, 2007, when the fare system was revised to categorize the service as a "bus" for fare purposes.
The "Washington Street" segment runs along various downtown streets, and mostly in dedicated bus lanes on Washington Street itself.
Two Washington Street routes start at Dudley Station in Roxbury and one terminates at Downtown Crossing on Temple Place (SL5-the 2002 route) and one to South Station on Essex Street (SL4)
The "Waterfront" section opened at the end of 2004, and connects South Station to Logan Airport with route SL1 via Ted Williams Tunnel and South Boston (Design Center area) with route SL2.
A new service to Chelsea opened April 21, 2018 via the same tunnel that SL1 uses and stops at the Airport Station of the Blue Line.
The buses that run the Waterfront section are 2004-05 dual-mode buses, trackless trolley in the Silver Line tunnel and diesel outside.
Service to Logan Airport began in June 2005.
The Waterfront segment is classified as a "subway" for fare purposes.
A transfer between segments is possible at South Station.

A "Phase III" tunneled segment was proposed to connect the two segments for through service, but it was controversial due to high cost and the fact that many did not consider Phase I to be adequate replacement service for the old Elevated.
 All Phase III tunneling proposals have been suspended due to lack of funds, as has the Urban Ring, which was intended to expand upon existing crosstown buses.

The MBTA contracts with private bus companies to provide subsidized service on certain routes outside of the usual fare structure.
These are known collectively as the HI-RIDE Commuter Bus service, and are not numbered or mapped in the same way as integral bus services.

Four routes connecting to Harvard Station (Red Line) still run as trackless trolleys; there was once a much larger trackless trolley system. (
See Trolleybuses in Greater Boston.)

In FY2005, there were on average 363,500 weekday boardings of MBTA-operated buses and trackless trolleys (not including the Silver Line), or 31.8% of the MBTA system.
Another 4,400 boardings (0.38%) occurred on subsidized bus routes operated by private carriers.

In June 2020 in the aftermath of COVID-19 pandemic, the MBTA had begun providing real-time
The MBTA bus system is the nation's sixth largest by ridership and comprises over 150 routes across the Greater Boston area. The area served by the MBTA's bus operations is somewhat larger than its subway and light rail service area, but is significantly smaller than that served by the MBTA's commuter rail operation. At least eight other regional transit authorities also provide bus services within that larger area, these being the Rhode Island Public Transit Authority, Brockton Area Transit Authority, Cape Ann Transportation Authority, Greater Attleboro Taunton Regional Transit Authority, Lowell Regional Transit Authority, Merrimack Valley Regional Transit Authority, Montachusett Regional Transit Authority, and Worcester Regional Transit Authority. All of these authorities have their own fare structures and some subcontract operation to private bus companies. In many cases, their buses serve as feeders to the MBTA commuter rail.
Within MBTA's bus service area, transfers from the subway are free if using a CharlieCard (for local buses); transfers to the subway require paying the difference between bus and the higher subway fare (for local buses; if not using a CharlieCard, full subway fare must be paid in addition to full bus fare). Bus-to-bus transfers (for local buses) are free unless paying cash. Many of the outlying routes run express along major highways to downtown. The buses are colored yellow on maps and in station decor.
The Silver Line is the MBTA's first service designated as bus rapid transit (BRT), even though it lacks many of the characteristics of bus rapid transit. The first segment began operations in 2002, replacing the 49 bus, which in turn replaced the Washington Street Elevated section of the Orange Line. A full subway fare was charged, with free transfers to the subways downtown until January 1, 2007, when the fare system was revised to categorize the service as a "bus" for fare purposes. The "Washington Street" segment runs along various downtown streets, and mostly in dedicated bus lanes on Washington Street itself. Two Washington Street routes start at Dudley Station in Roxbury and one terminates at Downtown Crossing on Temple Place (SL5-the 2002 route) and one to South Station on Essex Street (SL4)
The "Waterfront" section opened at the end of 2004, and connects South Station to Logan Airport with route SL1 via Ted Williams Tunnel and South Boston (Design Center area) with route SL2. A new service to Chelsea opened April 21, 2018 via the same tunnel that SL1 uses and stops at the Airport Station of the Blue Line. The buses that run the Waterfront section are 2004-05 dual-mode buses, trackless trolley in the Silver Line tunnel and diesel outside. Service to Logan Airport began in June 2005. The Waterfront segment is classified as a "subway" for fare purposes. A transfer between segments is possible at South Station.
A "Phase III" tunneled segment was proposed to connect the two segments for through service, but it was controversial due to high cost and the fact that many did not consider Phase I to be adequate replacement service for the old Elevated.  All Phase III tunneling proposals have been suspended due to lack of funds, as has the Urban Ring, which was intended to expand upon existing crosstown buses.
The MBTA contracts with private bus companies to provide subsidized service on certain routes outside of the usual fare structure. These are known collectively as the HI-RIDE Commuter Bus service, and are not numbered or mapped in the same way as integral bus services.
Four routes connecting to Harvard Station (Red Line) still run as trackless trolleys; there was once a much larger trackless trolley system. (See Trolleybuses in Greater Boston.)
In FY2005, there were on average 363,500 weekday boardings of MBTA-operated buses and trackless trolleys (not including the Silver Line), or 31.8% of the MBTA system. Another 4,400 boardings (0.38%) occurred on subsidized bus routes operated by private carriers.
In June 2020 in the aftermath of COVID-19 pandemic, the MBTA had begun providing real-time 
Massachusetts Bay Transportation Authority