Light rail is a mode of rail-based transport, usually urban in nature. When compared to heavy rail systems like commuter rail or rapid transit (subway), light rail systems are typically designed to carry fewer passengers and are capable of operating in mixed traffic or on routes that are not entirely grade-separated. Systems typically take one of four forms: the "first-generation" legacy systems, the "second-generation" modern light rail systems, streetcars, and hybrid rail systems. All of the systems use similar technologies, and some systems blur the lines between the different forms.

The San Diego Trolley is a light rail system operating in the metropolitan area of San Diego. The Trolley's operator, San Diego Trolley, Inc., is a subsidiary of the San Diego Metropolitan Transit System (MTS). The trolley operates as a critical component of the MTS, with connections to and integrated travel tickets with the local bus systems.