Different aspects of the presence of multiple copies of ribosomal operons in prokaryotic genomes are reviewed. Structure of prokaryotic ribosomal operons is briefly described. The available data are summarized regarding the copy number of ribosomal genes in various prokaryotic genomes, the degree of polymorphism of their individual copies, physiological and evolutionary aspects of the presence of the multiple copies of ribosomal genes. The review also considers the influence of the presence of multiple copies of ribosomal genes on the results of identification of prokaryotic isolates and of the studies of prokaryotic diversity in environmental samples based on phylogenetic analysis of 16S rRNA gene sequences.