Cryptosporidia and microsporidia are protozoan parasites that have emerged as human pathogens over the last two decades. With the coming of AIDS they have been recognised as important opportunistic pathogens of patients with an impaired host defence system. For both groups of pathogens, chemotherapeutic intervention is extremely limited. To date, cryptosporidiosis and microsporidiosis are severe and life-threatening opportunistic infections in AIDS patients and cryptosporidiosis is a major cause of diarrhea in infants and children in developing countries. This volume provides a comprehensive overview of the biology of the pathogens as well as the diagnosis and clinical aspects of the diseases they cause. The host immune responses against Cryptosporidium and the immunology of human microsporidiosis are reviewed in detail. Further topics discussed include the epidemiology and strain variation of Cryptosporidium parvum, the epidemiology and zoonotic aspects of microsporidia, and others.