Group therapy is used extensively within public mental health services, but more detailed knowledge is needed. All 25 health authorities in Norway were invited to describe their groups: theory, primary tasks, interventions, structure, patients and therapists. Four hundred twenty-six groups, 296 in community mental health centres and 130 in hospitals, were categorized into nine types, based on theoretical background. Psychodynamic groups were most frequent, followed by cognitive-behavioural, psycho-educative, social skills/coping and art/expressive groups. Weekly sessions of 90 min and treatment duration <6 or >12 months was most frequent. Main diagnosis for 2391 patients: depression (517), personality disorder (396), schizophrenia/psychosis (313) and social phobia (249). Patients with depression or personality disorder were mostly in psychodynamic groups, psychosis/bipolar disorder in psycho-educative groups. Cognitive-behavioural groups were used across several diagnoses. Most therapists were nurses, only 50% had a formal training in group therapy. There is a plethora of groups, some based on one theoretical school, while others integrate theory from several 'camps'. Patients with similar diagnosis were offered different group approaches, although some trends existed. More research evidence from regular clinical groups is needed, and clinician-researcher networks should be developed. More group therapists with formal training are needed.