Aminopeptidase N (APN)/CD13 is a type II metalloprotease that belongs to the M1 family of the MA clan, which consists of 967 amino acids with a short N-terminal cytoplasmic domain, a single transmembrane part, and a large cellular ectodomain containing the active site. APN has a molecular weight of 110,000. The APN exists in two forms, namely the membrane aminopeptidase N and the soluble aminopeptidase N. Moreover, it exhibits the presence of various isozymes with different functions. APN is a ubiquitous enzyme present in a wide variety of human organs, tissues and cell types (endothelial, epithelial, fibroblast, leukocyte). It is a multifunctional enzyme, related with tumorigenesis, immune system, pain etc. Furthermore, it also serves as a receptor for coronaviruses and other human viruses. Besides the manifestation of various other functions, APN is also involved in the trimming of antigen and the process of antigen presentation. These functions facilitate the modulation of bioactive peptide responses (pain management, vasopressin release) and influence immune functions and major biological events (cell proliferation, secretion, invasion, angiogenesis) thereby providing treatment options for many kinds of diseases. This review will introduce the structure and main functions of APN briefly.