Cathepsins (the term standing for “lysosomal proteolytic enzyme”) are a class of globular proteases that catalyze the hydrolysis of intracellular peptides and also exhibit extracellular functions. So far, a total of sixteen members of human cathepsin family have been identified, which are divided into three subfamilies, including cysteine protease family (cathepsin B, C, F, H, L, K, O, S, V, W, X and Z), serine carboxypeptidase family (cathepsin A and G) and aspartic protease family (cathepsin D and E). Cathepsins are highly expressed in various human cancers with each member playing different roles in distinct tumorigenic processes, including proliferation, angiogenesis, metastasis and invasion.