During the performance of a contract, changes in circumstances for one of the parties or an unforeseen issue may cause the completion of the contract to be difficult or impossible. Often, the parties will choose to alter the contract requirements to allow for these circumstances or waive their rights if a obligation is not met, while leaving the remaining contract obligations in place.

The most popular of these types of forms:

• Waiver of Default - this allows one party to a contract to waive or excuse a contractual default by the other party. This means that the non-defaulting party is choosing to forego pursuing all of its rights with regard to the default and to continue the contract as if the default did not occur. The remainder of the contract requirements will remain intact.
• Agreement Extending Time of Performance - Also referred to as an extension of performance, the sole purpose for this agreement is to extend the date in the contract when a specific event was to occur (e.g. the completion of construction, delivery of a good, etc.). Everything else in the contract will remain the same.