Functionally active 30s ribosomes can be reconstituted i n vitro from 16s RNA and a mixture of 305 ribosomal proteins under certain defined conditions. Our previous studies on the specificity and kinetics of reconstitution are summarized and discussed. The reconstitution reaction is first-order with respect to formation of active 305 ribosomes. The rate-limiting reaction is probably unimolecular, and it represents the structural rearrangement of an intermediate. Presumed reconstitution intermediates, or RI particles, have been isolated from reconstitution mixtures incubated at low temperature. It has been concluded that the reconstitution takes place in stepwise fashion: 16s RNA. --?->RI particles-+ RI* particles ------+ 305 ribosomes. New experimental results that show the highly cooperative nature of the assembly reaction are described; the number of sites, per RNA chain, which can bind ribosomal proteins independently from each other (i.e., without cooperativity) is at most two to three. Finally, another approach to the study of ribosome assembly i n vivo is described. It utilizes cold-sensitive E. coli mutants that are defective in ribosome biosynthesis at low temperature and accumulate incomplete "intermediate" ribosomal particles.