Pathogenic protozoa consist of various eukaryotic microorganisms which cause a number of parasitic diseases. Some of these diseases have clinical significance as they affect humans and include diseases like trichomoniasis, malaria, giardiasis, toxoplasmosis, amebiasis, leishmaniasis and Chagas disease. A prominent example of pathogenic protozoa are free living amoebae (FLA). They comprise opportunistic parasitic protozoa causing infections in humans as well as animals. FLA cause infections like amoebic encephalitis and Acanthamoeba keratitis. Protozoan pathogens can be identified in clinical samples by identifying the causative agent or by identifying particular immune reactions in the host. The diagnosis of the parasitic diseases caused by the protozoan pathogens is majorly dependent on microscopy. However, other methods such as techniques based on polymerase chain reaction (PCR) have also gained significance. This book aims to shed light on some of the unexplored aspects of protozoan pathogens and the recent researches on them. It is appropriate for students seeking detailed information on these pathogens as well as for experts.