In order to advance our understanding of why Results Based Financing (RBF) works or not, it iscrucial that evaluations not only measure the impact of such an arrangement on final outcomes(population health), but also assess the changes in variables in the causal chain betweenintervention and final outcomes. Health worker performance is a key variable in this chain; it isonly by changing health workers’ behaviors—their performance—that RBF can influence healthoutcomes. Careful assessment of impacts on health worker performance is therefore a naturaland important element of any RBF impact evaluations. This paper discusses various approachesto evaluating the impact of RBF on health worker performance. The first part is a discussion ofpossible ways in which RBF may affect health worker behavior, based on economic theory andempirical evidence. The second part is a more practical discussion of how health workerperformance and other relevant variables can be measured and how impacts can be estimated.This is followed by some practical steps that can be taken to ensure that the evaluation leads toactions that can be implemented; a brief conclusion completes the paper.