The plan for this atlas evolved from the necessity of providing the biology student interested in protozoology, cytology, and para­ sitology with an introduction to the study of fine structure in Protozoa. To reduce the book's extend, a selection of characteristic protozoans had to be made, limited to those which could be regarded as representative for entire groups. Interest in parasitic protozoans has been steadily on the increase over the last 10 years. This particular group of organisms thus seemed a very suitable choice. The" Apicomplexa" were selected as an area of emphasis. These once were part of the collective group called Sporozoa which included many parasitic protozoans of uncertain taxonomy. Fine structural research has been of especial significance for the Apicomplexa, since Protozoa belonging to this subgroup can now be named, characterized, and classified by features recognizable by electron microscopy. Only the fine structure of whole cells is represented in this atlas, so that the ciliates have been of necessity excluded. Their cells are too large in diameter in any case for our purpose here. They also play only a minor role as parasitic organisms. This book utilizes a new method to facilitate the analysis of protozoan fine structure. An electron micrograph, a descriptive text, and an analytic drawing are arranged on two facing pages so that the electron micrograph and the drawing can be compared.