Voluntary conservation agreements are becoming increasingly important in implementing the Endangered Species Act on private land. We analyze when such agreements arise and what level of conservation they generate in the presence of uncertainty about future government regulation and conservation benefits. Our results suggest that the likelihood of an agreement depends on the availability of assurances regarding future regulation. In particular, an agreement may not be reached if there is a high degree of uncertainty regarding future conservation requirements. The level of conservation attainable from an agreement depends on the likelihood of regulation, the bargaining power of the parties, the irreversibility of development, and the availability of assurances. Under conditions likely to hold in practice, a higher conservation level may be achieved by offering assurances. However, this level of conservation will not be optimal, and may be lower than that attainable from regulation.