The Chicago "L" (short for "elevated") is the rapid transit system serving the city of Chicago and some of its surrounding suburbs in the U.S. state of Illinois. It is 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), and the third busiest rail mass transit system in the United States after the New York City Subway and Washington Metro.

Chicago's "L" provides 24-hour service on some portions of its network, being one of only four rapid transit systems in the United States (the Chicago "L", the New York City Subway, PATH, and Philadelphia's PATCO Speedline) to do so. However, Chicago and New York City are the only cities in the US to offer 24 hour subway/elevated train service throughout their respective city limits every day (PATCO does not service throughout the city limits of Philadelphia). The oldest sections of the Chicago "L" started operations in 1892, making it the third-oldest rapid transit system in the Americas, after Boston's "T" and the elevated lines in New York City.