Mutual wills are wills made by two people, often spouses, in which each gives his/her estate to the other, or agree upon the distribution of their assets. If there is a contract in which each makes the will in the consideration for the other person making the will, the agreed upon dispositions of property are binding and a later change will invalidate
the will.

Mutual wills may contain an obligation not to revoke a will and thereby breach the agreement which is entered into at the time the mutual wills were made. Where one party has died the other will be obliged not to revoke his or her will subsequently. Where the surviving party attempts to do so a constructive trust will be imposed to remedy the injustice which might arise from breach of the original agreement.

The most common situation is where husband and wife to make wills leaving their property to the other, if the other survives, and in default to a beneficiary, but it is not essential that the surviving testator should receive property under the will of the first testator to die. If the spouse dies without having altered or revoked his will he has performed his part of the bargain and this creates an obligation on the part of the surving spouse to uphold the agreement.