Mucosal-associated invariant T (MAIT) cells are evolutionarily conserved T cells that are restricted by the non-classical major histocompatibility complex class-1b molecule MR1. MAIT cells recognize riboflavin (vitamin B2) derivatives in a MR1-dependent manner. Following antigen recongnition, MAIT cells rapidly produce Th1/Th17 cytokines, such as interferon-gamma and interleukin-17, in an innate-like manner. MAIT cells maintain an activated phenotype throughout the course of an infection, secrete inflammatory cytokines, and have the potential to directly kill infected cells, thus, playing an important role in controlling the host response. In this review, we discuss current knowledge regarding the role of MAIT cells in infectious diseases, cancers, and autoimmune diseases.