Parasitic amoebae of the genus Entamoeba are transmitted from one host to another as cysts. The active amoeboid forms are so adapted to life within their particular hosts that, outside the host, they soon die, but encysted stages survive conditions fatal to the unprotected amoebae. The cyst is an integral and critical phase in the life-cycle of Entamoeba; its attributes directly influence the dissemination of the organism, and are an important aspect in the epidemiology of amoebiasis. The following account of the morphology, formation and development of the cysts of Entamoeba is an assessment of present knowledge, and is not a comprehensive review of all available observations.