Trolleybuses, also known as trackless trolleys, take their power from electricity. They get the electricity from overhead wires via trolley poles. To complete the circuit, two poles are needed. There are currently about 315
MBTA Trolleybus (BW)

A low-floor trolleybus in Cambridge, Massachusetts. The left hand door is for operations in the Harvard Square lower busway, where the platform is on the left.

transit systems in 45 countries using trolleybuses. Some transit systems create trackless trolley routes to cut costs, or when a streetcar service doesn't have enough ridership to necessitate track maintenance. Uncommon in the United States, trolleybuses are mostly found in Europe, Russia, and China.

Trolleybuses are much quieter than other buses. Without the noise of an engine, the only noise comes from power steering or air conditioning. However, this also subjects people to what's known as the "silent death." Trolleybuses are also advantageous on hilly routes, since they provide better start-up force than regular buses on hills. Trolleybuses are also more environmentally friendly than other buses, since their power is produced more efficiently at centralised power plants.

Unfortunately, with routes subject to bus bunching, trolleybuses cannot overtake each other unless two separate wire pairs are provided. Re-routings are also bad for trolleybuses, since they are limited to the wires above. Dewirements, when the poles fall from the wire, can also happen. When it does, drivers are forced to get out and manually reconnect them. People also complain about the jumble of wires that can come up above an intersection.

Modern trolleybuses are not always constrained to wires. Cities like San Francisco and Vancouver have bought trolleybuses with batteries, allowing them to go fairly long distances from the wires. The MBTA in Boston uses dual-mode trolleybuses for its Silver Line Waterfront service. The buses start in a tunnel using electric power from wires, then turn to diesel power above ground.