A comprehensive search of the research literature was undertaken to explore the relationship between skill substitution and quality of care. It was possible to identify three central assumptions based upon the studies reviewed. The first suggests that a rich skill mix of mostly qualified nurses is often an ineffective and inefficient way of providing health care. The second assumption is based on research which appears to indicate that a skill mix of mostly unqualified staff is an ineffective approach to health care provision. Finally, there is a plethora of research findings which suggest that the most effective and efficient way to provide health care lies in the employment of high numbers of qualified staff. This paper examines the evidence for each of these positions and highlights the inherent limitations of the underlying studies.