The participation of ubiquitin in diverse fundamental cellular processes is presented. Its involvement in e.g. cell cycle control, DNA repair, regulation of development, stress response, ribosomal biogenesis, and proteolysis is described. Viral ubiquitin and ubiquitin-like proteins, linkage of ubiquitin gene loci, exon-intron boundaries, evolution of polyubiquitin genes, and the implication of nucleotide-identical positions of introns in ubiquitin genes for evolutionary aspects is discussed.