__Introduction: __ Assertive Community Treatment is a model for care and treatment of patients with the most severe mental illness in the community. Key principles of Assertive Community Treatment (ACT) are: integration of services, low patient–staff ratio, locus of contact in the community, medication management, focus on everyday problems in living, assertive outreach, and time unlimited services. ACT is the most extensively studied care delivery model for people with severe mental illness (SMI) and widely implemented in- and outside the US. Despite the extensive number of studies, there are still some research questions remained. As studies examining the association between the degree to which the ACT model is implemented and effect on patient outcomes are rare, the role of model fidelity is unclear. Also, it is unknown whether some ingredients of the model are more important than others, or whether certain ingredients are associated with specific patients outcomes. From 2005 on, another care delivery model for SMI patients was developed. Flexible Assertive Community Treatment, Flexible ACT, was inspired by and based on the ACT model, but with its adaptions more suitable in rural areas and able to serve a broader range of patients with severe mental illness. The Flexible ACT team is a case management team with partly an individual approach and partly a team approach; the approach varies from patient to patient, depending on the patient’s needs. For more stable long-term patients, Flexible ACT provides coordinated multidisciplinary treatment and care by individual case management. Unstable patients at risk of relapse, neglect and readmission are provided with intensive assertive outreach care by the same team, working with a shared caseload for this subgroup. __Aims of the thesis:__ This thesis addresses two principal aims: - To study the association between (ingredients of) the ACT model fidelity and patient outcomes; - To describe the development of the Flexible ACT scale. Chapters 2 to 6 of the thesis use ...