The 'L' (also written, "L", El, EL, or L) (from "elevated") is the rapid transit system serving the city of Chicago and some of its surrounding suburbs. It is operated by the Chicago Transit Authority (CTA). It is the second longest rapid transit system in total track mileage in the United States, after the New York City Subway, and is the third busiest rail mass transit system in the United States, after New York City and Washington, DC's Metro. Chicago's 'L' is one of four heavy-rail systems in the United States (CTA, MTA, PATH and the PATCO Speedline) that provides 24-hour service on at least some portions of their systems. The oldest sections of the 'L' started operating in 1892, making it the second-oldest rapid transit system in the Americas, after New York City. The 'L' has been credited with helping create the densely built-up city core that is one of Chicago's distinguishing features. The 'L' consists of eight rapid transit lines laid out in a spoke-hub distribution paradigm mainly focusing transit towards the Loop. Although the 'L' gained its nickname because large parts of the system are elevated, portions of the network are underground, at grade level, or open cut.

On average 722,782 people ride the 'L' each weekday, 483,177 each Saturday, and 414,512 each Sunday. Annual ridership for 2006 was 195.2 million, the highest since 1993. However, the CTA divides actual riders by roughly 1.2 to count riders who transfer between lines, putting the total number of riders at about 162.7 million. In a 2005 poll, Chicago Tribune readers voted it one of the "seven wonders of Chicago," behind the lakefront and Wrigley Field but ahead of Willis Tower (formerly the Sears Tower), the Water Tower, the University of Chicago, and the Museum of Science and Industry.