Many cathepsins are lysosomal proteases that play an important role in antigen presentation, tissue remodeling and enzyme activation/inactivation. They are also implicated in cancer, Alzheimer’s disease and rheumatoid arthritis. Structurally, they belong to different protease classes and families. Cathepsins B, C, H, L, S, V, X/Z/P, 1, 3 and 6 are cysteine proteases of the papain family. Cathepsins D and E are aspartic proteases of the pepsin family. Cathepsin A, a multifunctional enzyme, is a member of the serine carboxypeptidase family. Cathepsins are synthesized as inactive proenzymes and processed to become mature and active enzymes. Endogenous protein inhibitors, such as cystatins and serpins, inhibit active enzymes.