Considerable research effort has recently been directed to elucidating the structure of the chloroplast-the site of photosynthesis in plant cells. In 1952 Weier & Stocking (1) reviewed the structure of the chloroplast together with its biochemical and genetic aspects as it had been studied during the previous ten years. They concluded that not only was there a lack of neces sary experimental data, but also considerable confusion in the already exist ting data. Since 1952 improved techniques in microscopy, particularly in electron microscopy, chromatography, and other analytical methods have provided additional experimental information. This is evident from the numerous research papers and reviews on chloroplast structure and photo synthesis that have since appeared. Attention is therefore directed to the recent reviews by Frey-Wyssling (2), Granick (3), Mlihlethaler (4), and Thomas (5,6) j to Rabinowitch for a historical as well as tabular summary of the data (7, 8, 9); and to a symposium of the Brookhaven National Labora tories, The Photochemical Apparatus: Its Structure and Function (10). In addition to these reviews, reference should be made to Leyon (11), von Wettstein (12) and Heitz (13) for discussions of more specific structural aspects of chloroplast development. Also to be noted are the reports of two recent symposia in the U.S.S.R., one on The Origin of Life o n Earth (14), and the other the Second All Union Conference on Photosynthesis (15). One hesitates, in this active field of research, to attempt to assess all of the accumulated experimental data or to review the recent excellent reviews. In gathering the information for this review three questions were paramount. (a) Can the structure of the chloroplast now be discussed at a molecular level? (b) What are the new data on the composition of the chloroplast and on the synthesis of its proteins and pigments? (c) How do these data relate to the growth and structural development of the chloroplast? It was with these considerations in mind that the recent available literature was searched. The structure containing the photosynthetic pigments exists in a variety of shapes and sizes from the photosynthetic bacteria to the higher plants. These have been referred to as chromatophores, plastids, free grana, mega plasts and chloroplasts. This nomenclature was intended to give some idea of the plant photoreceptor structure as well as its phylogenetic position. Because of certain inconsistencies additional nomenclature has been intro duced and suggested. In this discussion of the research literature it was found desirable to call all of these structures chloroplasts unless otherwise indicated or defined.