Recent work on the circumsporozoite (CS) proteins of Plasmodium spp. is reviewed. It is concluded that CS proteins, and in particular the repeats, are not only targets of the host's immune response, but are also prime candidates for vaccine development. An understanding of the function of the repeat domains and of the genetic mechanisms that determine its stability within one strain of parasite while promoting variation among different strains or species, is likely to provide insight into important strategies for parasite invasion and survival.