When performance is not verifiable, firms in a long-term relationship may rely on incentive contracts that are self-enforced or “relational.” This paper studies the optimal design and performance of relational contracts in partnerships such as joint ventures or buyer-seller alliances. Optimal contracts look the same in each period as long as the relationship continues, but may require termination of the relationship after bad outcomes. Payments between the partners depend on their relative performance. In the special case of bilateral trade with specific investments, optimal relational contracting results in a price that varies with cost and demand conditions but is more stable than under spot market bargaining. Parallels are drawn with “Japanese style” subcontracting.