A Marine Expeditionary Force or MEF (formerly known as a Marine Amphibious Force) is the largest type of a Marine Air-Ground Task Force. A MEF is the largest building block of United States Marine Corps combat power.

An MEF is larger than a Marine Expeditionary Unit (MEU) or Marine Expeditionary Brigade (MEB).

Each MEF consists of a division as the ground combat element, an aircraft wing as the aviation combat element, and a logistics group as the logistics combat element.

The MEF also contains a Special Operations Training Group (SOTG) and is the training section component for the MEU and MEB. The SOTG oversees the training and evaluation exercises for the MEU's annual Special Operations Capable Certification.

A MEF also commands several smaller MAGTFs, including MEBs and MEUs.

Expeditionary Force is a generic name sometimes applied to a military force dispatched to fight in a foreign country. The term was particularly common in World War I and World War II. Military formations which have had Expeditionary Force in their title include: