Monoamine oxidase (MAO), a mitochondrial enzyme which metabolizes monoamines in both the central nervous system and peripheral tissues, plays an important role in the regulation of neuronal activities by controlling the concentrations of monoamines. During the past three decades the enzymological properties of MAO have been studied extensively. The most exciting and important advances in these studies have been the determination of the nucleotide sequence coding two forms of human MAO (MAO-A and MAO-B), and the involvement of MAO in potent dopaminergic neurotoxicity of MPTP, which had led to the acceleration of aetiological studies of Parkinson's disease. Other aspects of MAO studies have been the application of selective and safe MAO inhibitors in the treatment of depression and Parkinson's disease as well as the correlation between MAO and several psychiatric and neurodegenerative disorders.