In this article, Elmhurst will contest attorneys explain lack of testamentary capacity as grounds to invalidate an Illinois will or trust. As our Elmhurst will contest lawyers explain, in order to execute a valid will, the person creating the will or trust (“the testator”) must have the mental capacity to understand the ramifications of the act of executing the will or trust. Lack of testamentary capacity is one of several bases for arguing that a will or trust is invalid, including undue influence, fraud, revocation, and noncompliance with formalities of execution.