Contesting a will involves challenging the validity of the document after it has been submitted to the probate court for consideration. There are only a few grounds that are accepted for challenging a will. If the deceased can be shown to have been mentally incompetent at the time the will was created, it may be considered invalid. Likewise, if a third party can be shown to have coerced or "unduly influenced" the testator the will may be rejected. If the will was not properly signed and witnessed or if it is a fake it can also be thrown out.

Any person who has an interest in the will can contest it. This might be someone who believes he/she should have inherited and did not, or someone who did not inherit as much as expected. However, only the validity of the will can be challenged - not its perceived fairness. If a will contains a "no-contest" clause, the person contesting the will may be completely disinherited if he/she loses the case. Some states refuse to enforce a no-contest clause if the will is contested for "probable cause". This term means there was something obviously wrong with the will that made its validity questionable. A will may be voided in part or the whole document may be considered invalid depending on the circumstances of the case.