The use of means testing for determining eligibility has become increasingly popular in developing countries wishing to improve the targeting performance of their social safety net programs. However, past experience shows that means testing often reduces program participation of eligible households—this is true even for universally available programs in developed countries. High non-take-up rates reflect the important role that self-selection patterns can play in program participation levels by different socioeconomic groups. Yet there is still very little empirical evidence on the nature and magnitude of the trade-offs between program coverage of the eligible population and targeting performance, especially for developing countries. This paper contributes to filling this gap by evaluating the targeting performance of Mexico’s Oportunidades program.