Schizophrenia is clinically heterogeneous but it is not known whether this is due to the existence of discrete subtypes. For the purpose of explication, 'indicators' of schizophrenia are divided into three levels: phenomenology, pathophysiology, and aetiology. Five heterogeneity models and a number of quantitative approaches are described. It is imperative to apply rigorous methods of study to the comparison of unitary models and competing heterogeneity models of schizophrenia.