If you disagree with the terms of a will, your first thought may be to mount a legal challenge against it — but you have to have a valid reason. It isn’t enough that you simply disagree with its terms.

Here are the most common reasons that you might contest a will:

1. The deceased lacked testamentary capacity

In order for a will to be valid, the person making it must be able to understand what they are doing when they make the will. That includes understanding the value of their estate and the consequences of including — or excluding — someone from an inheritance. If the deceased had dementia or otherwise lacked the appropriate mental clarity to properly execute a will, that’s a good reason to challenge it.

2. You suspect that fraud was involved

Some people will simply take measures into their own hands when they don’t like what they see in a will. Sometimes an elderly person will be tricked into signing an “updated” will that doesn’t say what he or she believes it says. Other times, entire wills have been forged.

3. You believe that undue influence came into play

It’s unfortunate, but the elderly and infirm are often incredibly vulnerable to manipulation. If the deceased was isolated by someone prior to his or her death — and that person mysteriously ended up inheriting most of the deceased’s estate to the exclusion of all others — it’s wise to suspect some form of manipulation.

4. There are multiple wills in existence

Sometimes a deceased will make more than one will without destroying all the old copies of the previous will. If one of the previous wills is filed with the probate court, it may be necessary to contest it and prove that a later will should be used instead.

If you believe that a will is invalid for one of these reasons or for another reason, an attorney who handles will contests and probate litigation can help you determine if you have the proper standing to challenge the will.