This article provides a theory of interfirm partial ownership. We consider
a setting in which an upstream firm can make two alternative types of invest- ment: either specific investment that only a particular downstream firm can use
or general investment that any downstream firm is capable of using. When the
benefits from specific and general investments are both stochastic, equity partic-ipation by the downstream firm in the upstream firm can lead to more efficient outcomes than take-or-pay contracts. The optimal ownership stake of the down-stream firm is less than 50 percent under a natural assumption about relative bargaining power.