Protozoans are single-celled, eukaryotic organisms that often have complex life histories. Of the parasitic protozoans, only a small proportion is associated with disease and only a small percentage of these infects dogs. The few protozoans that infect dogs do not tend to be host-specific, but instead affect a variety of mammalian species. Notable among protozoans that infect both dogs and humans are Leishmania spp. which cause leishmaniasis, Trypanosoma cruzi which causes Chagas disease and Giardia intestinalis which causes giardiasis. This chapter focuses primarily on the transmission, diagnosis and treatment, prevention and control, zoonotic potential and public health considerations of these diseases.