The phylum Apicomplexa is characterized by the unique cell organisation of the zoites, the infective stages of unicellular parasites previously designated as Sporozoa. Apicomplexa includes Coccidian and Hematozoa well known for human and veterinary diseases they cause, such as malaria, toxoplasmosis, babesiosis, coccidiosis, and the large group of Gregarines, the early branching Apicomplexa. Gregarines are parasite of invertebrates and urochordates and they performed an extraordinary radiation from the marine and terrestrial hosts known from the Cambrian biodiversity explosion. After the basic publication in the Traite de Zoologie by Grasse in 1953, this second edition updates the knowledge with information provided by new technologies such as electron microscopy, biochemistry and molecular biology and to enlighten their high diversity of adaptation to invertebrate hosts living in a diversity of biotopes. The extracellular development of Gregarines, the considerable diversification of their cell cortex, their wide distribution in Annelids, Crustaceans, Echinoderms, Myriapods or Insects with about hundred thousands of species contribute to the understanding of many biological aspects of the pathogenic Apicomplexa. Since 1953, taxonomical reviews on Gregarines were published in the Illustrated Guide of Protozoa. In this supplement, there is a special emphasis on the hosts.