Changing skill mix is often identified as a potential solution to health services staffing and resourcing problems, or is related to health sector reform. This paper discusses what is meant by skill mix, provides a typology of the different approaches to assessing skill mix and examines, by means of case studies, the contextual, political, social and economic factors that play a part in determining skill mix. These factors are examined in relation to three factors: the reasons (or drivers) for examining skill mix; the impact of contextual constraints; and the effect of varying spans of managerial control. Case studies conducted in Costa Rica, Finland, Mexico, the UK and the USA are used to explore the reality of assessing skill in different contexts and health care settings. We argue that, although skill mix may be a universal challenge, it is not a challenge that all managers or health professionals can meet in the same way, or with the same resources. Context can have a significant effect on the ability of health service managers to assess and change skill mix. The key determinant is the extent to which these factors are in the locus of control of management nationally, regionally, or locally, within different countries. We emphasise the need to evaluate the problem and examine the context, before deciding if a change in skill mix is the answer. The local managerial span of control and degree of organisational flexibility will be major factors in determining the likely impact of any attempts to change skill mix. Before embarking on a skill mix review, any organisation should ask itself the question: 'If changing skill mix is the answer, what is the question?'