Study Aim: Despite ‘Assertive Outreach’ being a widely researched model of mental health delivery, previous research has mainly focused on how features of the service model encourage service users to engage with the service. Little is known about how the staff working in such services promote service user engagement, especially with service users who are particularly difficult to engage. This qualitative study explores how assertive outreach staff engage and maintain engagement with their service users. Design, Setting and Participants: Eight care co-ordinators were recruited from three assertive outreach teams in South East Wales. Participants were interviewed about how they engage and maintain engagement with service users. Transcribed interviews were analysed using Grounded Theory. Four of the participants also attended a subsequent focus group and discussed the preliminary analysis of the interview data. Findings: Three themes appeared to conceptualise the engagement process, ‘Building the therapeutic relationship’, ‘Maintaining the therapeutic relationship’, and ‘Service factors enabling engagement’. The first two conceptualise the personal factors (i.e. approaches, strategies and personal attributes) that staff use on a daily basis to engage and maintain engagement with service users. The final theme, ‘Service factors enabling engagement work’, focuses on the service related factors staff identify as being important in enabling them to engage with the people who use their service. Conclusion: Results from the current study provide a number of useful insights into the specific approaches, techniques and strategies used by assertive outreach staff to engage and maintain engagement with ‘hard to engage’ service users. The study also identified service elements and personal staff qualities that appear to facilitate the effective engaging of clients. Clinical implications are discussed and suggestions made regarding clinical practice and future research.