Diagnosis refers to developing the best methods for classifying disorders and for identifying their characteristic symptoms. Most importantly, however, diagnosis refers to the identification of particular disorders that differ in their underlying mechanisms and causes, and therefore defines them as discrete diseases. Most current work on schizophrenia attempts to identify the best ways to classify it and the best ways to identify and define its characteristic symptoms to improve the ongoing search for mechanisms and causes. One idea that is currently widely discussed is the distinction between positive or florid symptoms and negative or defect symptoms. Much more research needs to be done on this model, however, and we need many more studies that attempt to integrate biological research with the careful work that has been done to date on clinical description. The astute modern investigator must be able to move freely between the biotype and the phenomenotype if he or she is to understand fully the clinical picture and ultimately the cause of schizophrenia.