[2] The first trolleybuses were introduced in Milan as vehicles operating exclusivly within the 1906 world fair, at Piazza D'Armi, a plaza which has since been demolished and replaced by the Citylife district.

[3] In subsequent decades, the system developed rapidly, with the opening of radial and tangential routes.

The new external circular route (originally CE (circolare esterna), now 90/91) was built as a trolleybus line.

[1] In the mid-1970s, it was intended to transform the 90/91 circular route into a light rail line, and abandon the rest of the trolleybus network.

Currently, efforts are focused on reducing journey times, with the construction of dedicated lanes,[6] and the provision of higher quality busses,[7] which replaced all instances of the older trollybus models.