Monoamine oxidase (MAO; EC 1.4.3.4.) plays an essential role in the oxidative deamination of biogenic and food-derived amines, both in the central nervous system and the peripheral tissues. The enzyme was first described by Hare as tyramine oxidase [1] and was called monoamine oxidase by Zeller [2] in 1938. MAO is prevalent in animals [3]. The early findings in the late 1950s that iproniazid, with its tuberculostatic activity, caused a mood-elevating effect and inhibited MAO [4] gave an impetus to the clinical application of MAO inhibitors (MAOIs) in the treatment of depression and also to the search for new inhibitors. Their effects were not only beneficial in the treatment of psychiatric disorders, but they substantiated the concept that certain psychiatric states derive from the same neurochemical imbalance. Although the inhibitors proved to be as effective as the tricyclic antidepressants in the treatment of major depression, they fell into disrepute in the early 1960s because of the severe, and sometimes unpredictable interactions between MAOIs and food-derived amines (tyramine), which lead to cardiovascular complications and even death in some cases, and came to be called the “cheese effect” [5, 6]. In recent decades many efforts have been made to avoid the cheese effect. New MAOIs were developed with no or less severe adverse cardiovascular reactions [7,8].