Psychiatric inpatient services provide important care for individuals with serious mental health problems. Studies show that passivity and social disengagement prevail in inpatient settings, and the transition to outpatient care is associated with increased suicide risk. Behavioural Activation is an intervention that targets depression by increasing personally meaningful activities. Preliminary research shows that Behavioural Activation can be used in inpatient settings. The overall aim of this thesis was to evaluate the feasibility and effectiveness of Behavioural Activation for individuals in psychiatric inpatient settings, and in the transition between inpatient and outpatient care. Study I investigated inpatient activities and associated experiences. Study II was a pilot single-case experimental study of the feasibility and effectiveness of inpatient Behavioural Activation. Study III, was an interrupted time series evaluation of nursing-adapted Behavioural Activation across three wards. In Study IV Behavioural Activation in the transition from inpatient to outpatient care was compared to Supportive Therapy in a randomised controlled trial with 64 participants. The primary outcome was that of self-reported depressive symptoms and participants were followed up 12 months after treatment completion. Doing nothing was the most common inpatient activity, along with meal related activities. Passive and solitary activities were associated with negative distress and reward profiles. The preliminary evaluation of Behavioural Activation found high patient and staff satisfaction, and four of six participants showed improvement in depressive symptoms and functioning. After nursing-adapted Behavioural Activation was implemented on three wards, engagement increased. Avoidance decreased but later returned to baseline levels. Depressive symptoms and global clinical severity did not improve after nursing-adapted Behavioural Activations was introduced. The randomised controlled trial found that adding Behavioural Activation in the transition to outpatient care had a small, short-term, advantage over Supportive Therapy for self-reported depression. In conclusion, inpatient disengagement is associated with distress, and Behavioural Activation is a feasible intervention in inpatient settings that can be used by both trained therapists and nursing staff to increase patients’ treatment engagement. Behavioural Activation seems useful in targeting depressive symptoms in the transition from inpatient to outpatient care, a period associated with increased risk of suicide and clinical deterioration.