It is generally accepted that neuropsychiatric disorders have a biological basis. Pathological changes leading to illness have been confirmed for Alzheimer's (amyloid plaques) and Parkinson's disease (loss of dopaminergic transmission) and are the foundation for studies on the molecular biology of these disorders. For other neuropsychiatric disorders, in particular for schizophrenic and affective disorders, molecular causes appear to be more complex and therefore remain hypothetical, despite decades of research. Changes in a number of neuronal pathways and structures have been reported to be associated with these disorders and are currently under extensive investigation. The present volume reviews recent knowledge with emphasis on ongoing research findings. Current hypotheses based on these findings are described and discussed.