You can only contest a will for a valid legal reason — not simply because the outcome feels unfair. Here are the six grounds courts recognize and what each one requires.
Important: A successful contest usually needs (1) standing — you must be an "interested person" who would inherit if the will fell — and (2) at least one valid ground below, backed by evidence, filed before your state's deadline.
The will-maker did not understand what they owned, who their natural heirs were, or the act of making a will.
At the moment of signing, the testator must understand (a) the nature and extent of their property, (b) the people who would naturally inherit (the "natural objects of their bounty"), and (c) that they are making a will that distributes that property. Dementia, severe illness, medication, or a stroke around signing can support this ground. Evidence: medical records, physician testimony, the drafting attorney's notes, witness observations.
Someone in a position of trust overpowered the will-maker's free choice.
A person in a confidential relationship (caregiver, new spouse, adviser) uses pressure, isolation, or dependency so the will reflects their wishes, not the testator's. Courts look for: a confidential relationship + active involvement in procuring the will + an unnatural result (e.g., the influencer suddenly inherits most of the estate, longtime family is cut out). Evidence: isolation of the testator, who arranged/paid for the will, sudden beneficiary changes, the testator's vulnerability.
The will-maker was deceived into signing or into its terms.
Fraud in the execution (tricked about what the document is) or fraud in the inducement (lied to about facts that changed who they favored). You must show a deliberate false statement the testator relied on. Evidence: the false representations, who made them, and how they changed the will.
The will was not signed and witnessed the way state law requires.
Most states require the will to be in writing, signed by the testator (or at their direction), and witnessed by the required number of competent witnesses, all present as the statute requires. Missing or interested witnesses, no signature, or an unsigned page can invalidate the will. This is often the cleanest ground because it turns on the document, not someone's state of mind. Evidence: the will's signature/attestation page, the witnesses, any notarization.
The signature or the document itself is fake.
The testator's signature was forged, or pages were swapped/altered. Usually proven with a forensic handwriting (questioned-document) expert plus circumstances. Evidence: original document, known signature samples, expert analysis.
A newer valid will or codicil replaced this one, or the will was revoked.
A later validly-executed will or codicil can revoke an earlier one, and physical destruction with intent to revoke also counts. If the will being probated was superseded, it should not control. Evidence: the later document, proof of destruction, the testator's intent.
Disinheritance by itself is legal. You need a defect in capacity, process, or honesty — one of the six grounds above.