We propose to use modules representing functional subunits of the brain as the substrates of pathogenetic factors in mental disorders. This approach is based on scientific evidence from neurophysiology and cognitive psychology regarding the organisation of the human brain in functionally discernible, not necessarily temporally or spatially stable subunits, which are interconnected in complex, often multilayered networks of neuronal circuits. Such a definition would reconcile modern network theories of brain function with localizationist models. Module-based diagnosis and therapy of mental disorders will be the goal of a modular psychiatry, which has the advantage that it is founded in neuroscientific evidence and does not rely on arbitrary definitions of mental disorders based mainly on clinical empiricism.