Publisher Summary This chapter describes the structure, function, and evolution of archaeal ribosomes, with special emphasis on the rRNA (ribosomal ribo nucleic acid) and r-protein molecules that form complex macromolecules. The archaeal ribosomes appeared to contain features not present in bacterial ribosomal subunits, but which in turn, were less complex than the eucaryal ribosomes. Results from other laboratories confirmed the increasing complexity of the ribosomal morphology as one moved from bacteria to archaea and eucarya. The rRNA gene organization of the archaea is very variable and includes examples of organisms where the three rRNA genes are closely linked and cases where the three genes are unlinked. The number of rRNA operons is also variable. The r-proteins, like the rRNA, are important phylogenetic probes in the study of molecular evolution. In addition, comparative studies on the amino-acid sequence of these proteins often reveal regions that are highly conserved, suggesting these regions may have important functional roles. Protein sequence comparisons are a powerful tool to determine the evolutionary relationship between homologous proteins in the various organisms.