Although it is not common, the validity of a will can be challenged on numerous grounds. A successful challenge to a will can invalidate either a section of the will or the entire will itself. The most common grounds for contesting a will are:

Undue influence – the testator was improperly influenced by a close friend or caretaker to make unusually large distributions to that person
Lack of testamentary capacity – the testator did not have the required mental capacity to make a valid will
Insane delusion – the testator was suffering from a belief which he adhered to despite all evidence to the contrary and that belief materially affected a provision in the will
Fraud – the testator was intentionally misinformed about a matter relating to the distribution of his estate and this misinformation caused him to materially alter the way that he directed that distribution

In order to contest a will for any of these reasons a person must have standing. Essentially, standing is the right to take a legal action. In order to have standing to contest a will a person must either be named as a beneficiary in the will, or be eligible to inherit from the estate if the will or a provision of the will is deemed to not be valid. Contests and challenges require legal proceedings and can be long and costly.