Assertive outreach teams intended to engage clients with complex severe and enduring mental illnesses with services, are now common across the United Kingdom. Chapter one explores what is meant by the term 'engagement' and attempts to define the construct as it is used within assertive outreach. This chapter also gives consideration to the validity with which engagements currently measured. Chapter two, the main empirical paper, examines the psychometric properties of the current observer-rated measures of engagement used by assertive outreach teams. The results indicated that whilst the measures were all found to be reliable, there were a number of differences between the measures in terms of their construct validity. Little is known about client experiences of services in relation to engagement, with a number of methodological limitations identified with the few qualitative studies that have been conducted. Chapter three reports on the findings of a focus group study with clients under the care of an assertive outreach team. Whilst generally more satisfied with the assertive outreach approach, the participants identified a number of tensions regarding service delivery, which if addressed, may enable teams to further facilitate client engagement. The model of assertive outreach calls for clinical psychologists to work in increasingly non-traditional ways, with a strong emphasis on forming flexible and open relationships with clients. Items in the current engagement measures concerning the relationship between client and clinician have been shown to be related to outcome. Furthermore, the findings of qualitative studies report that clients find the relationship they have with individual team members pivotal for their engagement with services. Chapter four, therefore, reflects on the required changes to traditional professional boundaries for clinical psychologists working within assertive outreach teams and the impact of these changes on their working relationships with both clients and colleagues.