Inspired by facts from the private-sector construction industry, we develop a model that explains many stylized facts of procurement contracts. The buyer in our model incurs a cost of providing a comprehensive design and is faced with a tradeoff between providing incentives and reducing ex post transaction costs due to costly renegotiation. We show that cost-phis contracts are preferred to fixed-price contracts when a project is more complex. Me briefly discuss how fixed-price or cost-plus contracts might be preferred to other incentive contracts. Finally, our model provides some microfoundations for ideas from Transaction Cost Economics.