Parasites experience two environments; one reflecting external conditions, the other created by the living host. The constraints imposed by these environments provide an avenue into the rich diversity of parasitology. Papers review familiar and unfamiliar extreme physical conditions from low temperatures and desiccation to the powerful water currents faced by some fish parasites. The environment created by the host and parasite adaptation to host immunity is covered in several papers, including immune evasion, host-switching, and the effect of parasites on the evolution of immunity.