The paper provides a quantitative analysis of the factors that determine successful implementation of IMF-supported programs. To this end, we construct new measures of program implementation and compliance with conditionality for 170 IMF programs approved between 1992 and 1998. The main hypothesis tested is whether IMF effort and the design of conditionality significantly affect the probability of successful implementation of IMF-supported programs. We find that program implementation depends primarily on the borrower’s domestic political economy. Strong special interests in the parliament, political instability, inefficient bureaucracies, lack of political cohesion, and ethno-linguistic divisions weaken program implementation. IMF effort, the design of conditionality, and initial and external conditions do not materially influence program prospects.