An outline is presented of four characteristically different types of partial hospitalization programs currently operating in the Netherlands. The findings are based on empirical data gathered from a study of 85 institutions active in this particular field. The institutions are distinguished according to the function they perform within the total mental health care system, the assumption being that these programs are capable of fulfilling four main functions: 1) an alternative to full-time hospitalization; 2) a continuation of full-time hospitalization; 3) an extension of ambulatory treatment; 4) a day care or rehabilitation service for the chronically mental ill. A study was made of the differences and similarities between four types of institution specializing in one of the four functions mentioned above. In this connection, the authors have particularly concentrated on staffing, number of places, accommodation, organization, referral, patient population (age, education, diagnoses), treatment (type, duration, drug use) and contraindications.