Increasingly, financial incentives are being used in health care as a result of increasing demand for health care coupled with fiscal pressures. Financial incentive schemes are one approach by which the system may incentivize providers of health care to improve productivity and/or adapt to better quality provision. Pay for performance (P4P) is an example of a financial incentive which seeks to link providers' payments to some measure of performance. This paper provides a discussion of the theoretical underpinnings of P4P, gives an overview of the health P4P evidence base, and provide a detailed case study of a particularly large scheme from the English National Health Service. Lessons are then drawn from the evidence base. Overall, we find that the evidence for the effectiveness of P4P for improving quality of care in primary care is mixed. This is to some extent due to the fact that the P4P schemes used in primary care are also mixed. There are many different schemes that incentivize different aspects of care in different ways and in different settings, making evaluation problematic. The Quality and Outcomes Framework in the United Kingdom is the largest example of P4P in primary care. Evidence suggests incentivized quality initially improved following the introduction of the Quality and Outcomes Framework, but this was short-lived. If P4P in primary care is to have a long-term future, the question about scheme effectiveness (perhaps incorporating the identification and assessment of potential risk factors) needs to be answered robustly. This would require that new schemes be designed from the onset to support their evaluation: control and treatment groups, coupled with before and after data.