Within organisations there is a growth in the use of team development interventions designed to enhance effective team performance. This had been particularly notable in senior management and leadership teams. However this development has not been underpinned by rigorous, organisationally based research (West and Slater, 1995) and the research which has been undertaken has not adequately addressed the issues (Lorsch, 1989; Higgs et al., 2005). It is therefore important to undertake systematic empirical research to underpin the validity of such interventions. The relationship between top team performance and organisational performance was given a clear focus and a sense of direction as long ago as the 1980s by Hambrick and Mason's 'Upper Echelon Theory' (Hambrick and Mason, 1984). This promoted a stream of demographically based research, but the results have been limited and sometimes contradictory (Pettigrew, 1992; Higgs et al., 2005), in part as a result of the absence of direct data relating to the impact of teams on performance. Also both team and group research have failed to analyse the importance of the mix of personalities and team processes in achieving high performance (Higgs, 1999). This paper presents the results of a research programme involving fifty-four senior management teams using direct data. The research assesses the importance of the mix of personalities in a team and the processes they employ in working together, in determining performance outcomes. The results highlight the benefits of successful team processes, many of which relate also to Higgs and Dulewicz's studies of board processes (1997). This finding indicates the potential value of process intervention as a means of developing team performance.