We analyze the problem of a buyer who purchases a long-term project from one of several suppliers. A changing state of the world influences the costs of the suppliers. We distinguish between complete contracts conditioning on all future realizations of the state of the world and incomplete contracts renegotiated whenever the state of the world changes. We provide conditions such that incomplete contracting does not pose a problem. If the changing state of the world is publicly observable and the buyer cannot switch between suppliers during the lifetime of the project, the buyer achieves the same surplus irrespective of whether contracts are complete or incomplete. An English auction followed by renegotiation whenever the state of the world changes is optimal. To identify conditions when buyers should consider drafting complete contracts, we extend the analysis by considering private information about the changing state of the world and supplier switching. In both cases, incomplete contracting poses a problem. In a survey of procurement consultants, we confirm that publicly observable states of the world via price indexes play an important role in procurement. Moreover, the consultants confirm that supplier switching is infrequent in procurement practice. Thus, incomplete contracting is less of a problem in a considerable share of procurement projects. However, complete contracts are useful and could be used more often.