Protozoan parasites compromise an extremely diverse group of eukaryotic life forms. The Phyla Apicomplexa and Microsporidia are obligatory parasites. All Microsporidian parasites are intracellular in host cells and most Apicomplexan parasites live inside of host cells. These two Phyla of protists contain some of the most interesting and diverse groups of parasites infecting vertebrates and invertebrates. Opportunistic Infections: Toxoplasma, Sarcocystis, and Microsporidia will focus on two important Genera of Apicomplexan parasites, Toxoplasma gondii and Sarcocystis species, and the medically important members of the Phylum Microsporida. We have been fortunate in obtaining excellent contributions from many experts in the field. Volumes in the World Class Parasites book series are written for researchers, students and scholars who enjoy reading about excellent research on problems of global significance. Each volume focuses on a parasite, or group of parasites, that has a major impact on human health, or agricultural productivity, and against which we have no satisfactory defense. The volumes are intended to supplement more formal texts that cover taxonomy, life cycles, morphology