Detection and determination of proteinases, studies on their interactions with substrates and effectors and the investigation of their physiological role are greatly facilitated by the use of proteinase inhibitors. In this context, especially synthetic, low-molecular weight inhibitors of different selectivity are very useful. They are widely applied during purification and characterization of proteinases. Furthermore, synthetic inhibitors are useful tools for suppression of undesired proteolytic activity. Depending upon the manner in which the inhibitor is attached to the enzyme, one distinguishes reversible and irreversible inhibitors.