In this paper the authors consider how psychological coaching can help those with managerial responsibility develop crucial managerial diagnostic skills. The paper refers to three organisational models that coaching psychologists could find helpful as they seek to help managers accurately diagnose the causes of organisational problems. These organisational models can be labelled as follows: (1) Systems approach; (2) Unitary and Pluralist Perspectives; and (3) Role Behaviours versus Personality Behaviours.The paper also explains how lack of effective diagnosis can lead those with managerial responsibilities to get involved in a range of activities better left to others. Given that organisations operate as systems (Millett, 1998) faulty diagnosis can arise because of a lack of understanding that problems which emerge in one function of an organisation may be caused by actions in other parts of an organisation. Managers need to anticipate the ‘knock-on’ effect that their decisions can have on other areas of organisational activity. Further causes of defective diagnosis explored include a failure of managers to distinguish between role and personality behaviour and the adoption by managers of a unitary as opposed to a pluralist frame of reference, leading to an inability to recognise conflicts of interest and deal with these in a constructive way. Practical advice is given on the development by coaching psychologists of managers’ diagnostic skills related to these models with reference to case studies.