Especially during the last decade, there has been a steady development of psychiatric day treatment in the Netherlands. The extent, function and position of this mode of treatment in the framework of the Mental health system, have until now not been sufficiently determined. An overview of the international literature is presented, concerning a range of items such as the treated population, program-evaluation, comparisons between day treatment programs, the relative effectiveness of day treatment versus 24-hour- and outpatient treatment, and the predictive factors for a successful treatment. Percentage of rehospitalization, the burden on the family and the financial factors are also being dealt with. The choice of material is limited to non-selective day treatment for adult psychiatric patients. The article seeks to provide up-dated scientific information in aid of the ongoing discussion about the substitution principle.