We investigate the idea that linear contracts are reliable because they give the same incentives for eort at every point along the contract. We ask whether this reliability leads to a microfoundation for linear contracts, when the principal is prot-maximizing. We consider a principal-agent model with risk neutrality and limited liability, in which the agent observes the realization of a mean-zero shock to output before choosing how much eort to exert. We show that such a model can indeed provide a foundation for reliable contracts, and illustrate what elements are required. In particular, we must assume that the principal knows a lower bound, but not an upper bound, on the shocks.