If your will isn't valid, it literally isn't worth the paper it's written on -- which means that your personal possessions and assets will be distributed according to the way the state sees fit, not the way that you necessarily wanted.
What makes a will valid in Illinois?
-- You have to be a legal adult. In Illinois, that means you must be aged 18 or older.
-- You have to be of sound mind. This essentially means that you understand what a will actually is and what it accomplishes. Wills often get disputed based on the testator's mental capacity -- which has led some people to include doctor's letters among their estate documents, written close to the time of their wills, attesting that they were of sound mind at the time their wills were created (especially if they expect someone in the family to be angry over their will).
-- You have to have the actual intention to make a will. In other words, you didn't think you were signing some other legal document, like a power of attorney.
-- Your actions must be voluntary. A lot of wills are disputed based on whether or not the testator signed under duress, was somehow coerced or was under someone's exceptionally strong and manipulative influence.
-- You have to properly dispose of all your assets and personal items. This could be done very simply or could include an incredibly detailed list.
-- You need to sign and date your will in front of two disinterested witnesses. That means that the witnesses can't be any of your heirs.
-- Your witnesses have to be credible. In other words, would they be able to say how they knew you were the person signing the will? Did they know you personally or check your identification? Do they have any reason to lie? Are they believable?
-- It must not be a holographic will, which is one that is handwritten but lacks witnesses. A handwritten will with the required witnesses is acceptable.
-- It cannot be a nuncupative will, or one given verbally, even if there were credible witnesses there to hear it.
The best way to make sure that your will is valid (and avoid unpleasant estate litigation) is to have an attorney help you prepare one. If you need to make or revise your will, let your attorney know.
Source: FindLaw, "What Is a 'Valid Will'?," accessed April 25, 2017