MAO A and MAO B follow the same chemical mechanism to oxidise primary, secondary, and tertiary amines, but they are distinguished by differences in their substrate and inhibitor specificities and in their kinetic behaviour. Studies on the purified enzymes show that monoamine oxidases are unusual enzymes because certain amine substrates accelerate the oxidative half-reaction (much more in A than in B) and because substrate binding induces an enormous positive shift in the redox potential of the flavin. The molecular basis of these features is still unknown, as is the structure of the active site, information necessary for national drug design. This article reviews the biochemistry of MAO in general and speculates about what the kinetic and thermodynamic properties observed in the isolated enzymes mean for the catalytic expression of their amine oxidase activities in vivo. Specific and distinct physiological roles for MAO A and MAO B are probable because they are expressed in different proportions in different cell types and their expression varies in development and ageing. Molecular localization techniques can now be used to measure the levels of MAO in specific cell types rather than a region of mixed cells. When this information is combined with the kinetic constants, it will be possible to construct numerical models to predict the metabolism of a given amine by a particular cell which should help in understanding the role of amines in development and perhaps also the role of the endogenous inhibitors in altering levels of bioactive amines.