The biochemistry and biological function of the naturally occurring polyamines, putrescine, spermidine, and spermine, have been reviewed with special reference to animal organisms. These compounds are universally distributed in all living material. Their biosynthesis from ornithine and methionine is accurately controlled and may fluctuate according to the metabolic needs of the cell. Polyamines strongly and specifically interact with nucleic acids in vitro. It appears that under physiological conditions a substantial portion of cellular polyamines is noncovalently bound to nucleic acids and nucleic acid-containing structures such as ribosomes. Polyamines are able to stimulate protein and ribonucleic acid synthesis in vitro. In several systems characterized by rapid growth polyamines and ribonucleic acid accumulate in parrallel. Evidence that polyamines may have an essential role in protein and/or nucleic acid synthesis is substantiated by recent observations on polyamine-deficient bacterial mutants, although no specific function has been established with certainty as yet. Some clinical applications of polyamine research related to cancer are also discussed briefly.