Prenuptial agreements are contracts between spouses, typically entered into before the marriage. After the marriage, spouses can enter into what are known as post-nuptial agreements. As contracts, they are binding on each spouse, just like any other contract.
Most of the time, unless there are normal defenses to a contract—such as fraud or unconscionability—the agreement will be enforceable. However, you may encounter difficulties enforcing provisions related to custody, visitation, or child support. Courts will always make determinations based on the best interests of the child at the time, which may not have been predictable when the agreement was made before marriage.
Provisions regarding property division and spousal support are typically enforceable.