There is a lot of interest in how well financial incentives influence the delivery of health care. Financial incentives are extrinsic sources of motivation and they exist when an individual can expect a monetary transfer which is made conditional on acting in a particular way. Since there are several reviews describing the effects of different types of financial incentives, it is important to bring this together in an overview to examine which are best at changing healthcare professionals' behaviours and what happens to patients. We therefore conducted an overview of systematic reviews that evaluated the impact of financial incentives on healthcare professional behaviour and patient outcomes. We searched a wide range of electronic databases from when they started up to December 2008. We included systematic reviews of studies evaluating the effectiveness of any type of financial incentive. We grouped financial incentives into five groups: payment for working for a specified time period; payment for each service, episode or visit; payment for providing care for a patient or specific population; payment for providing a pre-specified level or providing a change in activity or quality of care; and mixed or other systems. We summarised data using vote counting. We identified four reviews reporting on 32 studies. Two reviews were of moderate quality and two were of high quality. The studies that the reviews reported on were of low to moderate quality. Payment for working for a specified time period was generally ineffective. Payment for each service, episode or visit was generally effective, as were payment for providing care for a patient or specific population and payment for providing a pre-specified level or providing a change in activity or quality of care; mixed and other systems were of mixed effectiveness. When looking at the effect of financial incentives overall across different outcomes, they were of mixed effectiveness on consultation or visit rates; generally effective in improving processes of care, referrals and admissions, and prescribing costs; and generally ineffective in improving compliance with guidelines outcomes. On the basis of these findings, we concluded that financial incentives may be effective in changing healthcare professional practice. The evidence has serious methodological limitations and is also very limited in its completeness and generalisability. We found no evidence that financial incentives can improve patient outcomes.