It's rare, but wills can be challenged in court and tossed out for a number of reasons.

Will challenges are very unusual -- by one estimate, about 99% of wills sail through probate without a hitch -- but they do happen. If a will doesn't fulfill certain legal requirements, or the maker of the will was not of age or sound mind, someone who stands to benefit from getting the will thrown out can challenge it in probate court after the will-maker's death. There are several grounds a would-be heir or beneficiary can base a legal challenge on.

Age

It's almost never an issue, but the maker of the will must have been:

• 18 years of age or older, or
• living in one of the few states that permit younger persons to make a will if they're married, in the military, or otherwise considered "emancipated."

Mental State

The maker of the will must have been of "sound mind" when the will was made. This is not a rigorous requirement. Usually, a court faced with resolving a question of mental capacity requires only that the person who made the will:

• knew what a will does and that knew that he or she was making one
• knew who he or she would normally be expected to provide for, such as a spouse or children
• understood what he or she owned, and
• was able to decide how to distribute his or her property.

In reality, a person must have been pretty far gone before a court will rule a will invalid. Forgetfulness or even the inability to recognize friends doesn't, by itself, establish incapacity.

Fraud or Undue Influence

A will can also be declared invalid if a court determines that it was procured by fraud, forgery, or "undue influence." This usually involves some evil-doer who occupies a position of trust -- for example, a caregiver or adult child -- manipulating a vulnerable person to leave all, or most, of his property to the manipulator.

Contents of the Will

What makes a document a valid will? For starters, every state has rules about what a will must, at a minimum, contain. Most states require that the document:

• expressly state that it's the will of the person who wrote it
• include at least one substantive provision, such as a clause leaving some property to someone or appointing a personal guardian for a minor child, and
• appoint an executor (called a "personal representative" in some states), the person responsible for carrying out the terms of the will when the time comes. Nevertheless, in most states, even if an executor is not named, the court will appoint one and then enforce the will.

Witnesses

A typed or computer-printed will must have been dated and signed in the presence of at least two adult witnesses. Three witnesses are required in Vermont. In most states, the witnesses cannot be people who are named to inherit property under the will. (If a witness inherits, this may void the gift to himself but not the rest of the will.)

Handwritten, unwitnessed wills are valid in about half the states. These "holographic" wills must be written and signed entirely in the handwriting of the person making the will. (Some states, but not all, require that they be dated.) Because there are no witnesses, holographic wills are more easily challenged than standard typewritten wills; the probate court must be satisfied that the document is actually in the deceased person's handwriting and was intended to serve as a will.

Notarization

Wills don't have to be notarized to be valid. Some wills, however, include a "self-proving" affidavit (sworn statement) that the witnesses sign in front of a notary public, which means the witnesses don't later have to come to court to swear that the will is valid.

The Will-Maker's Residence

Generally, a will is valid in any state where the maker of the will died, if it was valid under the laws of the state (or country) where the maker of the will was "domiciled" when the will was made. A person's domicile is the state the person considered his or her permanent home -- where a person's principal home was and where the person spent the most time, as opposed, say, to a summer home.