Collagen is the most abundant vertebrate protein and forms a stable fibrous architecture in connective tissues, such as bone, cartilage, skin and tendon. Much recent research has been directed towards an understanding of the molecular and cellular mechanisms involved in the synthesis and degradation of collagen, because a change in the normal balance, or an increased destruction of collagen, can cause loss of function of specialized tissues. This short review attempts to summarize present knowledge about the proteolytic destruction of collagen (...) and the exciting new work on the factors that are emerging as important controlling agents. An understanding of the regulatory processes is yielding some important clues regarding the levels at which they can fail in pathological conditions and we speculate about evolving strategies for preventing uncontrolled resorption. (shrink)