Increasing interest in attaining an optimal health have longstanding and are imperative for consumers, medical and health researchers and functional food scientists and producers among others. Functional food is defined as foods that have a biological function with a very broad range of coverage including natural and conventional foods with reported bioactivities, dietary supplements and medical foods. Its concept is inclusively broad and ambiguous. Medical food is a specific class of functional foods with clear definition and limited coverage to guarantee its integrity. This review introduced the definition of medical food by the US Food and Drug Administration, intended to clarify the differences among medical food, dietary supplements and conventional foods, and listed some examples of medical food products developed in the US.