will Contests, Generally
Someone who contests or challenges a will must generally prove that the will being offered for probate is not a valid expression of the decedent's wishes.  Typical allegations of invalidity involve improper execution, lack of sound mind, fraud, and undue influence.

However, will contests are not always an attempt to prove that the will is invalid. For instance, a child born after a parent makes a will may have certain rights to the parent's estate, but cannot assert those rights without contesting the parent's will.

See: Will Contests, Who May Contest; Sound Mind, Generally; Improper Execution, Generally; Fraud, Generally; Children Born After A Will; Children Born Out of Wedlock