Cyanopyrrolidines (cyanopyrrolidides, pyrrolidine-2-nitriles, prolinenitriles) as inhibitors of the serine protease dipeptidyl peptidase IV (DPP-IV, DP IV, CD26, EC 22.214.171.124) were first reported in 1995. The interest in this compound class grew immensely when DPP-IV was discovered as a target for the treatment of type 2 diabetes. The research on cyanopyrrolidines cumulated in the discoveries of vildagliptin (LAF237, NVP-LAF237) and saxagliptin (BMS-477118). These compounds entered Phase III clinical trials in 2004 and 2005, respectively, and an application for market approval has been filed for vildagliptin in 2006. Today cyanopyrrolidines are, as judged by the numbers of patent applications, the most prominent of several series of DPP-IV inhibitors, and have the potential to become valuable medicines for type 2 diabetes in the near future. This review summarizes some historical aspects of the discovery of cyanopyrrolidine DPP-IV inhibitors, and then focuses mainly on structure-activity-relationships, the evolution of different subseries, the possibilities to improve on the chemical instability that is associated with this compound class, and on the discoveries of vildagliptin and saxagliptin. Within this context, the properties of individual compounds and results from biological studies are discussed. The rationale of DPP-IV inhibition, clinical data, and the relevance of selectivity over related proteases are extensively reviewed in other contributions to this issue of Curr. Top. Med. Chem., and are therefore only very briefly touched..