Despite historical assumptions to the contrary, there is little evidence that the majority of recognized mental disorders are separated by natural boundaries. Diagnostic categories defined by their clinical syndromes should be regarded as 'valid' only if they have been shown to be truly discrete entities. Most diagnostic concepts in psychiatry have not been demonstrated to be valid in this sense, though many possess 'utility' by virtue of the information they convey about presenting symptoms, outcome, treatment response and, in some instances, aetiology. While researchers in genetics, neurobiology and population epidemiology are increasingly more likely to adopt a continuum/dimensional view of the variation in symptomatology, clinicians prefer to hold on to the categorical approach embodied in current classifications such as ICD-10 and DSM-5. Both points of view have plausible justification in their respective contexts, but the way forward may be in their conceptual reconciliation.