The semicarbazide-sensitive amine oxidases (SSAOs) comprise a substantial but diffuse group of enzymes separable from classical monoamine oxidase in several respects. Differences in cofactor requirement, molecular weight and subcellular distribution are crucial for such a separation. Differential sensitivity to enzyme inhibitors, characterized by resistance to inhibition by acetylenic MAO inhibitors coupled with sensitivity to semicarbazide and some related compounds are characteristic of these enzymes. SSAO enzymes have been found in the plasma of man, ox, pig and horse, for example as well as in the solid tissues of many species. Extensive studies have so far failed to produce any conclusive evidence to indicate what the precise functions of many of these enzymes may be. Indeed in most cases there is no clear idea as to the nature of the preferred physiological substrate, although many amines with pharmacological activity have been shown to be substrates. The actions of these amines may be potentiated following inhibition of SSAO, but as yet little is known whether or not these actions can be important in vivo. An attempt is made in this review to bring together some of the evidence to see if there are indications for future endeavours.