This paper analyzes optimal foreign aid policy in a neoclassical growth framework with a conflict of interest between the donor and the recipient government. Aid conditionality is modeled as a limited enforceable dynamic contract. We define the contract to be self-enforcing if, at any point in time, the conditions imposed on aid funds are supportable by the threat of a permanent aid cutoff from then onward. Quantitative results show that optimal self-enforcing conditional aid strongly stimulates the developing economy and substantially increases welfare. However, aid effectiveness comes at a high cost: to ensure enforceability, less benevolent political regimes receive permanently larger aid funds in return for a less intense conditionality.