Metropolitan Area Express (MAX) is a light rail system in the Portland metropolitan area in the U.S. state of Oregon. It is operated by TriMet and currently has three lines: Blue, Red and Yellow, with Green under construction and Orange planned.

As of late 2007, MAX is the fifth most ridden light rail system in the United States and the country's second most ridden standalone light rail system, after San Diego's. (The busiest light rail systems—Boston, San Francisco, and Los Angeles—are part of larger subway networks.) Since its inception about $3 billion has been invested in light rail in Portland.

In central Portland and Hillsboro, MAX trains run in reserved lanes on surface streets, unlike the Portland Streetcar, which runs in mixed traffic. Outside the central city, MAX runs primarily within its own right-of-way fully separated from traffic.

Some track segments are controlled by traffic signals at intersections, including the original segment along East Burnside Street and the newest segment along Interstate Ave. Other segments have gated crossings to give trains the right-of-way. One three mile (4.8 km) section is tunnel below Washington Park. While this section has only one station, it is 260 feet (79 m) below ground level, making it the deepest train station in North America and the second deepest train station in the world.

Because of Portland's relatively small 200-foot (61 m) downtown blocks, trains operate with only one or two cars. The MAX cars are about 90 feet (27.5 m) long, so a stopped train consisting of more than two cars would block intersections. Blue Line trains are typically two cars long. Trains on the Yellow and Red Line run in both one and two car configurations, though increasingly more Red and Yellow Line trains are two-car.