The nature and development of teamwork in primary health care is discussed and some barriers to effective teamwork identified. Theories of team effectiveness are outlined and methods of applying these to primary health care teams are explored. The 'constituency approach' and the theory of team innovation are examined in more detail. Team-building interventions are described as one way of improving team effectiveness but their limitations are also acknowledged. The Health Education Authority primary health care team workshops are used as an example of a team-building intervention. Research at the MRC/ESRC Social and Applied Psychology Unit, Sheffield University, is described. This is evaluating the outcomes of the primary health care team workshops in terms of team effectiveness. The research will measure viability (the extent to which the team sufficiently sustains good relationships to continue working together) and team performance (achievement of desired outcomes) pre and post workshop. Team effectiveness will be measured through team self-assessment, patient satisfaction and family health service authority designated targets for health promotion. The overall aim is to develop a model of team effectiveness for primary health care teams, which can then be used to diagnose and guide teams in their work.