This thesis provides knowledge of occupational engagement in day centres for people with psychiatric disabilities and contributes to measurement development in the area of occupational engagement. The first study focused on the characteristics of the occupations carried out at the day centre by exploring occupational engagement using time-use diaries with a person-environment-occupation perspective. The occupations performed were categorized as social occupations, maintenance occupations, creative occupations, manufacturing occupations, service occupations and information-focused occupations. The study also showed that being at the day centre meant participating in occupations with varying levels of demands on the individual. The second study highlighted that attendance in day centres brought a feeling of being socially included through participation in occupations. Attendance in day centres created structure and routines, and the perceptions of contributing and being entrusted with responsibilities were considered motivational. The attendees were challenged to learn new things, to be more active and to master new goals in their daily life, and were in a process of seizing control over their everyday lives. The third study was a psychometric study of the Profile of Occupational Engagement in people with Severe mental illness - Productive occupations (POES-P). The findings indicated good internal consistency, and a logical pattern of relationships between the POES-P and other instruments was found. POES-P was considered to be mainly a construct in its own right. The distribution of responses indicated a ceiling effect, which calls for further development of the instrument. Study IV revealed a stepwise indication of which factors are most likely to be important for occupational engagement. The participants’ psychopathology, especially general symptoms, and time spent at the day centre were important factors for reaching occupational engagement above a medium level, and self-mastery was the only variable associated with belonging to the group with the highest level (> the 75th percentile) of occupational engagement