A will can be revoked by the intentional physical destruction of the will or by the execution of a subsequent will. However, the testator must be of sound mind at the time either of these events occurs to accomplish the revocation.
In some cases, the law "forces" a revocation. For example, marriage revokes a prior will of either spouse in most states. In some states, divorce, annulment or the birth of a child also revokes a prior will.