Conversion from procollagen to collagen is a specific process that is a requirement for proper alignment of collagen molecules to form functional fibers. This process is catalyzed by at least three structurally and functionally distinct enzymes cleaving collagen types I-III. The cleavage processes possibly taking place in the more recently discovered collagen types are not known to any extent at this time. Two amino-terminal proteinases, one cleaving type I and type II procollagens and the other cleaving type III procollagen, have been purified close to homogeneity, and the more unspecific activity of carboxy-terminal proteinase has been isolated from several tissues. In our experimental model, however, cleavage of the carboxy-terminal propeptides of types I and III procollagen is differently affected by lysine. This suggests the presence of at least two distinct enzymes for the removal of carboxyl-terminal propeptides. The regulation of the reaction process from procollagen to collagen is not well known at present. The importance of the phenomenon in terms of fibril formation, however, is demonstrated by several elegant studies in vitro; and certain genetic disorders in which this process is defective demonstrate the significance in vivo. Moreover, the factors shown to effect the cleavage process may be potentially beneficial in the treatment of the pathological processes with abnormal collagen accumulation such as fibrosis. In this paper we briefly review the current knowledge of the converting enzymes, including some very recent findings of our laboratory as well as the evidence presented for the biological significance of the conversion process.