The number of psychiatric day hospitals is steadily increasing, thus finishing the dualism of in-patient treatment and out-patient treatment. This article informs about the characteristics of patients treated in psychiatric day hospitals, about patients' and their relatives' assessment of treatment, the effectiveness of psychiatric treatment in a day hospital and the cost-effectiveness of day-hospital treatment as compared to in-patient treatment. The study of the most important older publications and the most recently published randomized controlled trials shows the following: first, a great variety of different groups of patients are cared for in this setting, including, among others, mentally ill delinquents, gerontopsychiatric patients, addicted patients and patients with psychosomatic or personality disorders. Within the general psychiatric setting, most day-hospital patients seem to suffer from an at least moderately severe, often chronic disorder. Concerning socio-demographic characteristics of this group, the high unemployment rate is most remarkable. Second, patients' as well as relatives' responses to surveys about satisfaction with treatment are very positive, placing special emphasis on the experienced usefulness of the given structuring of the day, the experienced comradeship, the ongoing contact to the social environment, and the promotion of the patient's autonomy. Patients having already been treated in a day hospital assess this setting even more positive than patients with no such experience. Third, treatment effectiveness is supported by older publications as well as by recent findings of randomized controlled trials: as regards the most important outcome measures (e.g. reduction of symptoms, social functioning, quality of life, burden on relatives), no differences between day-hospital treatment and in-patient treatment could be found. However, day-hospital treatment seems to last significantly longer than in-patient treatment. Fourth, first studies on the cost-effectiveness of day-hospital treatment have found that it might lead to essential cost savings compared to in-patient treatment. However, altogether, with regard to the scientific state of the art, it still has to be stated that the number of empirical studies is yet limited, with randomized controlled trials only being conducted in few centres in the Netherlands, the UK and the USA, i.e. in countries with different approaches to acute day-hospital care. Thus, several suggestions for further research are made, giving top priority to a multi-centre randomized controlled trial using the most common set of outcome measures and carefully describing the applied methods and untoward events. As concerns the practical implications of the available findings, the authors draw the following conclusions should the reported findings be replicated by further studies: first, the capacity of available day-hospital places should be expanded. Second, existing facilities should think about the necessity and the possibility of an internal restructuring of the provided care. Third, as regards the in-service training of young psychiatrists, it might be reasonable to oblige them to practice within this setting for some time, in order to learn its specific characteristics.