Chaperonin homologs are found in all extant species, including eubacteria, archaebacteria, and eukaryotes. In bacteria and eukaryotic cell organelles, such as mitochondria and chloroplasts, chaperonin (cpn) 10 and cpn60 are present. In contrast to the above, no proteins closely related to cpn60 (or cpn 10) have been found in the archaebacteria and the eukaryotic cell cytosol thus far. These species instead contain members of a protein family referred to as TCP-1 which is distantly related to the cpn60 family of proteins. This chapter examines the cpn10, cpn60, and TCP-1 families of proteins from an evolutionary perspective to understand how these proteins have evolved, and their possible relationship to one another. The sequence data on these proteins also provide valuable information concerning the origins of the eukaryotic cell and eukaryotic cell organelles. Some of these aspects have also been reviewed in this chapter.