Testamentary capacity refers to a person's ability to understand and execute a will. As a general rule, most people who are over the age of eighteen are thought to be competent to make and sign the will. They must be able to understand that they are signing the will, they must understand the nature of the property being affected by the will, and they must remember and understand who is affected by the will. These are simple burdens to meet. However, there are a number of reasons a person might challenge a will based on testamentary capacity.

If the testator of a will suffers from paranoid delusions, he or she may make changes to a testamentary document based on beliefs that have no basis in reality. If a disinherited heir can show that a testator suffered from such insane delusions when the changes were made, he or she can have the will invalidated. Similarly a person suffering from dementia or Alzheimer's disease may be declared unfit to make a will. If a person suffers from a mental or physical disability that prevents them from understanding from understanding that a will is an instrument that is meant to direct how assets are to be distributed in the event of his or her death, that person is not capable of executing a valid will.

It is not entirely uncommon that disinherited heirs complain that a caretaker or a new acquaintance brainwashed the testator into changing his or her will. This is not an accusation of incapacity to make the will, but rather a claim of undue influence. If the third party suggested making the changes, if the third party threatened to withhold care if the will was not changed, or if the third party did anything at all to produce a will that would not be the testator's intent absent that influence, the will may be set aside for undue influence. Regardless of the reason for the challenge, these determinations will only be made after the testator's death if the will is presented to a court and challenged. For this reason, it is especially important for the testator to be as thorough as possible in making an estate plan and making sure that any changes are made with the assistance of an experienced estate planning attorney.