Adequate and reliable methods for evaluation of clients with work disabilities are crucial for both the individuals who are assessed and for society as a whole. Sound and precise work assessments are needed to guide clients to suitable interventions using a minimum of rehabilitation resources. Occupational therapy literature contains evidence that work function assessments are complex and that there is confusion over work assessment concepts. Therefore, further development and evaluation of adequate concepts and theoretical models within the area of work assessment is needed. This article proposes a conceptual framework for different dimensions of work functioning and points out important factors for work assessment. The concepts proposed and defined in the article are: work functioning, work participation, work performance, and individual capacity.