The main aims of clinical psychiatry are: A To cure people with mental and emotional disturbances, or to provide them help to cope with their emotional and environmental problems. B To provide data as exact as possible as a material for psychopathology to build hypotheses and theories to broaden and to deepen our insight in the problems of mental and emotional disturbances. C To procure facts as a basis for designs for effectave preventive measures together with other disciplines. The former and latter aims are closely interrelated, and cannot be viewed separately: Therapeutic help is based on psychopathological theories and on empirical clinical facts and psychopathology can only be fruitfully developed in close contact with the empirical facts from the clinical field. Therapeutic intervention on the one hand and psychopathological theories on the other have, in fact, preventive aspects. How do the facts and theories developed so far concerning heredity fit in this picture of clinical psychiatry? A—(1) Designs for therapy are always based — even when the therapist denies it on theoretical grounds — on diagnosis in one form or another. Diagnosis in this sense is not to be confused with categorizing or classifying the patient in nosological categories or giving him a place in a taxonomic system. Rather, the diagnostic procedure, resulting in an individual structural diagnosis, comprises two actions: (1) The assesment of the mode of experiencing and behaviour of the patient and ordering them in symptomatic and syndromatic patterns. (2) After the syndrome has been determined the etiological factors must be analysed in terras of a triad of hereditary, organic and psycho(-socio-)genic factors. Heredity can be interpreted as conditioning a particular disposition towards, or conditioning a vulnerability for mental and emotional disorders. Heredity may condition: a particular somatic and mental constitution; b the integrative capacity of the individual, (including the capacity to master tensions); c biochemical defects in the tentral nervous system. a and b may or may not be combined with signs of an imbalance of the autonomic nervous system or an abnormality of brain waves. A—(2) In view of therapy it must be stressed, that 'genetically conditioned' is not synonymous with 'incurable'. Inherited traits may give indications for the therapeutic measures to be taken. B— From a theoretical psychopathological point of view inheritance can be interpreted as a necessary but, in most cases, not a sufficient condition for mental and emotional disturbances. The genotype provides the preconditions upon which the favourable or injurious environmental factors can exercise their influence. Heredity is the first link in a chain of processes of interaction between genotype and environment. Men tal and emotional disturbances are generally not transmitted as such, but only the tendency towards or the susceptibility for these disorders. This tendency shows a broad variance in penetrante and expressivity. The gamut comprises such extremes as Chorea Huntington on the one hand and a slight emotional lability on the other. Some facts and theories concerning mental def iciency, chronic brain disorders, manic-depressive psychoais and schizophrenia will be presented. C— For the prevention of mental and emotional disturbances an exact knowledge of the genetic make up of the new-boen or the young child, but also of the adult will give us — in principle at least — the possibility for designing active measures. These measures have reference to the somatic, the emotional and the social fields.