No organic substrate is known for endogenous psychoses, neuroses and behaviour disorders. Therefore genetic studies have to depart from the behavioural level. This approach has yielded the empirical risk figures which depend on genetic and environmental factors. Beyond this level behaviour genetics endeavours to clarify the genetic mechanisms underlying normal and abnormal behaviour. In the schizophrenias f.i. several stretegies have been developed to approach the genetic basis: Mathematial models as to the mode of inheritance; use of modified classificatory and diagnostic criteria for genetic analysis; search for phenomena representing steps between genotype and phenotype and showing a clear mode of inheritance, f.i. biochemical or electrophysiological characters. In case the neurotransmitters are not changed in quality or total quantity but in compartmentation in the genetic defect might be localized in the pre- or postsynaptic membranes or receptors. Genetic factors are also involved in nonpsychotic behaviour disorders but to a lesser degree. They can be demonstrated even in psychoses with somatic background.