With rapid increases in healthcare spending over recent years, health economic evaluation might be thought to be increasing in importance to decision-makers. Such evaluations are designed to inform the efficient management of healthcare resources. However, research into health policy decisions often report, at best, moderate use of economic evaluation information, especially at the local level of administration. Little attention seems to have been given to the question of why economic evaluations have been underused and why they may yield different results in different contexts. There are many barriers to applying economic evaluations in situations which combine complexity with uncertainty. These barriers call for innovative and creative responses to economic evaluation of healthcare interventions. One response is to view economic evaluations in the context of complex adaptive systems theory. Such theory offers a conceptual framework that takes into account contextual factors, multiple input and output, multiple perspectives and uncertainty involved in healthcare interventions. This article illustrates how complexity theory can enrich and broaden policy-makers' understanding of why economic evaluations have not always been as successful as health economists would have hoped. It argues for health economists to emphasise contextual knowledge and relativist understanding of decision contexts rather than seeking more technically sound evidence-based reviews including economic evaluations.