Cysteine proteinases of protozoan and helminth parasites are considered to have a high potential as targets for novel antiparasite agents. This has stimulated research on the enzymes and a large body of data has now been accumulated. This Perspective provides an overview of the current situation, with recent advances being highlighted. Emphasis is given to the Type I cysteine proteinases of trypanosomatid protozoa, which are atypical in having an extra C-terminal domain, and the asparaginyl endopeptidase ofSchistosoma, for which the only homologues known are those in plants. The locations of parasite cysteine proteinases are described, with emphasis on the extralysosomal sites, and the putative roles of the enzymes in host-parasite interactions, including parasite nutrition and host invasion, are discussed. The major advances being made in elucidating cysteine proteinase function in protozoan parasites through genetic manipulation, including targeted gene deletion, are presented, with the Type I cysteine proteinases ofLeishmania being used as an example. The importance of this approach in the validation of the enzymes as drug targets is discussed. The current status of attempts to exploit parasite cysteine proteinases with drugs is presented, and future prospects are outlined.