This thesis concerns the contribution of team coaching to team effectiveness. The rise of team working in organisations has led to a growth of team coaching as an area of practice within organisations but the links between team coaching and team effectiveness are under-theorised. This study addresses this gap in the literature, focusing on whether team coaching makes a difference to team effectiveness, and if so, how. This study deploys an action research approach. Qualitative data were gathered over a 12-month period involving a team coaching process and three action research cycles. The study draws on the experiences of three different types of teams, their organisational stakeholders and the team coaches themselves. This led to two main methodological contributions to published team effectiveness and team coaching knowledge: the deployment of a qualitative action research process to address issues of team effectiveness and a multi-stakeholder analysis that gives ‘voice’ to organisational stakeholders and team members from different types of teams in the conceptualisation of team effectiveness in relation to team coaching practices. The findings and analysis make four main contributions to our knowledge and understanding of the relationship between team coaching and team effectiveness. First, that team coaching influences intra-team and team-stakeholder relationships and by extension the perceptions of team effectiveness by team stakeholders, resulting in increased engagement and visibility with stakeholders. The role of relationships as a feature of perceptions of team effectiveness also expands the established measures of team effectiveness to include the quality of team-stakeholder relationships. Second, a new understanding of the relationship between team coaching, team adaptation and team effectiveness, whereby team coaching is found to be a process that supports the team adaptation cycle, resulting in greater team effectiveness. Third, that team task and interdependence explain variations in team coaching and team effectiveness outcomes because of their influence on the importance of intra-team and team-stakeholder relationships on perceptions of team effectiveness. Fourth, the importance of the relational aspect of team coaching, conceptualised in the thesis as team coaching working alliance, for creating psychological safety within the team as a foundation from which change and adaptation can occur. The study thus provides a basis from which to evaluate both the efficacy of team coaching processes and the contribution of team coaching to team effectiveness.