Blastocystis is a polymorphic enteric protist which is associated with acute or chronic gastrointestinal illness which infects humans and animals and is prevalent mostly in developing countries. Blastocystosis infection is common in Pakistan and other countries with poor sanitation, low level of hygiene and socioeconomic conditions. The book which is based on Dissertation submitted in University of Karachi, Pakistan in 2013 involves the study on 450 patients with gastrointestinal problems. It gives an insight of the pattern of Blastocystis infection, detection of Blastocystis by different methods and antigen detection by ELISA, role of zoonotic infection in different hosts and relationship of Blastocystis in cancer patients. The book will be of immense value for researchers, scientists, doctors, paramedics and zoologist. It will also be useful for general public and research students to give an insight of a very common but neglected disease of the developing countries. Research into Blastocystis spp. should continue to progress in different aspects and applied to the study of this interesting protozoan parasite.