The article draws on an Economic and Social Research Council (ESRC)-funded research project that aimed to investigate the reality behind the rhetoric of "joined up thinking". The research project was a qualitative, multi-method study involving three phases, including observation and documentary analysis; interviews; and focus groups around decision making and knowledge sharing. The article reflects on the perspectives and experiences of health professionals and their colleagues in multi-agency teams about the impact of multi-agency teamwork on their professional knowledge and learning, and on their ways of working. Actual and potential conflicts between professionals are explored about models of understanding, about roles, identities, status and power, about information sharing, and around links with other agencies. Dilemmas of team building and of conflicting values and knowledge are exemplified from health professionals' accounts, using theoretical models of "communities of practice" and "activity theory". The article presents groups of strategies that health professionals and their colleagues in multi-agency, multi-professional teams use to overcome barriers and to strengthen team cohesion. The conclusion reflects on some implications of our findings in theory and practice for professionalism within integrated, multi-professional teams that are building new ways of working.