Macrophage migration inhibitory factor (MIF) is a classic pro-inflammatory factor that helps control both innate and adaptive immune responses. The expression of MIF is much higher in many tumor tissues, which helps the tumor grow, spread, make new blood vessels, and create an immune microenvironment that helps the tumor. Due to the important role MIF plays in the development and growth of tumors, it is being looked at as a possible way to treat tumors. This review discusses what MIF is, where it is found, how it sends signals, what it does in inflammation and tumors, and how drugs that target MIF are being made.