Ribosomes are multicomponent particles on which biosynthesis of proteins occurs in all organisms. The best known ribosome, namely that of Escherichia coli, consists of three RNAs and 53 different proteins. All proteins have been isolated and characterized by chemical, physical, and immunological methods. The primary sequences of 47 E. coli ribosomal proteins have so far been determined. Studies of the shape, as well as the secondary and tertiary structure, of the proteins are in progress.Various techniques (e.g. immune electron microscopy and cross-linking of neighbouring components in situ) give information about the architecture of the ribosomal particle. The first technique resulted in illustrative and detailed knowledge not only on the shape of the ribosomal subunits but also about the location of many proteins on the surface of the particles. The analysis of cross-links between ribosomal proteins and (or) RNAs has in several cases been pursued to the level of elucidating which amino acids and (or) nucleotides are cross-linked together in situ. Reconstitution of a fully active E. coli 50S ribosomal subunit from its isolated RNA and protein components can be accomplished by means of a two-step incubation procedure. From the analysis of the intermediates occurring during the reconstitution process it has been concluded that the in vitro reconstitution process resembles the in vivo assembly of 50S subunits in many respects. Escherichia coli mutants with alterations in almost all ribosomal proteins have been isolated. Their biochemical and genetic analyses are very useful tools for obtaining information about the structure, function, and biosynthesis of ribosomes as well as about the location of the genes for these proteins on the chromosome. From comparative electrophoretic, immunological, protein–chemical, and reconstitution studies on ribosomes from various species it has become clear that there is little homology between ribosomes from prokaryotes and those from eukaryotes. This finding is surprising since there is no essential difference in the way in which pro- and eu-karyotic ribosomes function in protein biosynthesis.