Title: Incompleteness as a Constraint in Contract Design Authors: Stefan Krasa and Steven R. Williams Abstract: We develop a model that encompasses both the incomplete contracts that are used in practice and the idealized complete contracts that address all contingencies. The objectives of the paper are (i) to examine the extent of the inefficiency caused by the constraint of contractual incompleteness, and (ii) to identify properties of agents' preferences that determine whether or not incompleteness causes inefficiency.