When should a buyer use a fixed-price contract versus cost-reimbursable?

Use a fixed-price contract when the scope of work is well-defined and unlikely to change, the seller can reasonably estimate costs, and you want cost certainty with minimal buyer risk. Use a cost-reimbursable contract when the scope is uncertain or likely to evolve, such as research and development or early-phase design, when the buyer needs flexibility to direct the work, and when no seller will accept the risk of a fixed price due to uncertainty.