Despite the fact that along the last 100 years a number of diagnostic systems and clinical models of schizophrenia have been proposed, defining this psychiatric disorder in a manner acceptable in both clinical and research settings remains one of the most difficult tasks in psychiatry. While the description of symptoms and signs of the disorder has remained mainly unchanged over the years, the way in which the authors have articulated the varied phenomenological manifestations has been different across periods. Great strides that have been made in recent years have brought about a convergence on a definition of the illness as well as methods of assessment that are acceptable to workers from research centres across the world. The structured interviews brought an organizational focus to the traditional psychiatric interview that provided a strict format and directions to the interviewer, which enables systematic assessment of all the criteria necessary for a diagnosis according to the given diagnostic system. Having a consensually shared set of diagnostic criteria as well as structured assessment devices helped to ensure a more common unit of analysis in research across the world. Though most workers would hail the operationalization of diagnostic criteria as an advance in psychiatry, there remain concerns about the impact that the quest for increased diagnostic reliability might have on validity. Given that the ultimate goal of any diagnostic system is to-provide insights into the aetiology, pathophysiology and treatment of the disorder, examining various alternative diagnostic constructs and their validity is still an important area of schizophrenia research.