The historical background of the Scandinavian concept of reactive psychoses is presented. The reactivity concept consists of several elements. There has to be a traumatic life event of psychological or somatic nature but its duration can be discussed. What is the time lag between the life event and the psychotic reaction? Some say one week, others up to one year. Is a personal disposition (vulnerability) a necessary condition for the psychotic reaction? Are emotional turmoil or non-organic confusion specific symptoms of the reactive psychotic reaction? Does the psychotic reaction have a meaning as an escape or a defense? What is the duration of the psychosis in relation to the duration of the life event? Does reactive psychoses always imply full recovery (good outcome)? The various editions of the ICD- and the DSM-classification include some of these features in their definition of reactive psychoses. Scandinavian experts also emphasize various aspects in their descriptions of these psychoses. Studies have shown that reactive psychoses can be diagnosed reliably. Work on the operatinalization of "reactivity" is under way. In the Scandinavian tradition there is a diagnostic shift from reactive psychoses to schizophrenia in later admissions. Generally, the reactive psychoses have a good outcome, and the outcome is significantly better than for schizophrenia and as good as for manic-depressive psychoses. Most cases of reactive psychoses are treated with neuroleptic drugs for some months. Electroconvulsive treatment is rarely given for reactive psychoses. Scandinavian psychiatrists think that DSM-IV and ICD-10 have not taken the concept of reactive psychoses seriously, and that research work has to be done to convince international psychiatry of the value of the concept.