The Chicago "L" is the rapid transit system serving the city of Chicago and some of its surrounding suburbs in the U.S. state of Illinois. Operated by the Chicago Transit Authority (CTA), it is the fourth-largest rapid transit system in the United States in terms of total route length, at 102.8 miles (165.4 km) long as of 2014, and the third-busiest rapid transit system in the United States. In 2016, the "L" had 1,492 rail cars, eight different routes, and 145 train stations. In 2023, the system had 117,447,000 rides, or about 373,800 per weekday in the fourth quarter of 2023.

Rapid transit or mass rapid transit (MRT), also known as metro, is a type of high-capacity public transport that is generally built in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be called a subway, tube, or underground. Unlike buses or trams, rapid transit systems are railways, usually electric, that operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles. They are often grade-separated in tunnels or on elevated railways.