A cost-plus contract is an agreement containing the promise to pay a contractor its total costs plus a percentage of the costs (the "plus") as profit.
The profit (or fee) may be set ahead of time or may fluctuate depending on the contract.
These types of contracts, if the profit is not fixed ahead of time, can be a perverse incentive for a contractor to run up costs as a means of increasing profit. As such, when used in the United States Government, the fee is set ahead of time and changes only if a change in the scope of work takes place.
Also in the US Government, a contractor working on a cost-plus contract is not required to continue work once funding limits are met and if no further funding is provided (except usually for a final report); this contrasts with the "fixed-price" contract where a contractor must deliver the end item regardless of final costs required to do so. As such, these contracts are most common in research and development activities.