This thesis research tested active user involvement in service design teams and explored its impact on the design process and its outcomes. It was found that not only the form of user involvement matters in service design but that particularly the characteristics of the involved users can impact on team performance and the development of design ideas in terms of user value, feasibility, and originality. The findings lead to a closer understanding of how co-design affects service design outcomes and what variations can occur during the design process when users are involved.