TransMilenio is a bus rapid transit (BRT) system that serves Bogotá, the capital of Colombia, and Soacha, a neighbouring city.

TransMilenio consists of several interconnected BRT lines, with raised floor stations in the center of a main avenue, or "troncal".

An additional set of 869[2] regular buses, known as "feeders" (alimentadores in Spanish), carry passengers from certain important stations to many different locations that the main route does not reach.

[3] Before TransMilenio, Bogotá's mass transit "system" consisted of thousands of independently operated and uncoordinated mini buses.

When Enrique Peñalosa was elected mayor he cancelled these projects and oversaw the construction of the initial TransMilenio system at a fraction of the cost.

The operational design of TransMilenio was undertaken by transport consultants Steer Davies Gleave with the financial structuring of the project led by Capitalcorp S.A., a local investment bank.

Bogotá won the first Sustainable Transport Award in 2005 due to the BRT system and urban cycling strategy.

On May 2 and 3, 2006, several groups of bus drivers not associated with TransMilenio held a strike, protesting against some elements and consequences of the system.

They disagreed with the amount of monetary compensation that they would receive in exchange for the disposal of old buses (10 to more than 20 years old), traffic restrictions on the TransMilenio main lines, and a new Pico y Placa Ambiental in some city areas, that would restrict the schedules of buses older than 10 years to early morning hours to reduce pollution in the city.

Due to the relatively high price, overcrowding, and delays in the routes, hundreds of people, mostly students protesting and some vandals looted and broke windows on March 9, 2012, causing half a million dollars of damage and 11 injuries.

A lawsuit by disabled user Daniel Bermúdez caused a ruling that all feeder systems must comply with easy access regulations by 2004, but this has not happened yet.

According to a United States Transportation Research Board (TRB) case study report, the initial construction cost for the first phase of 41 km was US$240 million, or US$5.9 million/km.

[22] These protests sometimes devolved into riots involving heavy police presence and the use of crowd control measures such as tear gas and water cannons.

[23] The system was described by users, independent bodies and the media as suffering from overcrowding with an average of eight passengers per square meter,[24][25] insecurity[26][27] and providing bad customer service.

[29] During construction, there were problems with the concrete used to pave the dedicated roads, which had an estimated cost to the city of 1.6 trillion pesos (500 million dollars).

[30] In 2015 a study made by the National University of Colombia revealed that 70% of the air pollution near Transmilenio exits was caused by the buses of the first phase.

[35] Several policies have been adopted in order to confront this problem, like an exclusive bus for women,[36] and special undercover policewomen.