Declining response rates threaten the validity of surveys. Much research has therefore been done on means to counteract this trend, such as advance letters, number of contact attempts, and giving incentives to interviewers or respondents. This chapter presents the theoretical assumptions behind the use of such measures, and focuses on the different kinds of incentives (i.e. prepaid vs. conditional, monetary vs. in-kind) and their effects on response rate and data quality in different survey settings. Special attention is given to the need to integrate such measures in the overall survey design. Future challenges for research on incentive effects are discussed.