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 selfenforcing 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.Foreign aid has been a substantial source of income in developing economies. As an example consider the case of Africa: on average aid funds amount to 11% of recipient's GDP.1 The stated intention of development assistance programs is to reduce poverty and to promote economic growth. However, the stagnating growth pattern in African countries calls the effectiveness of aid into question. 2 This paper analyzes optimal foreign aid policy in a neoclassical growth framework with a conflict of interest between the donor and the recipient government. We take into account that, instead of implementing economic policies that coincide with the donor's intention, the recipient government may follow 'bad' policies and use aid funds e.g. to assist political supporters or to finance military interventions. To prevent the government from doing so, it has become a common donor policy to impose conditions that specilY how foreign aid funds should be allocated. However, since the recipient country is sovereign, its government may not be willing to keep the conditions. The focus of this paper is to analyze the optimal design of incentive compatible aid conditionality. We follow the recent literature on limited commitment in dynamic macroeconomic settings and describe aid conditionality as an imperfectly enforceable dynamic contract between the donor and the recipient country.3 To ensure that the recipient government coop-