The general definition of a holographic will is that it is written by hand and is not witnessed. Different states have varying requirements for accepting these, and some do not accept them at all. In Illinois, the legality of a holographic will depends not on the handwriting but on the witnesses.
Chapter 5/4-1 of Section 755 of Illinois’ Code requires that all wills be in writing. Oral, or nuncupative, wills are not accepted. The testator, or the person making the will, must also be at least 18 years of age and have sufficient mental capacity to know what he is doing. If all legal criteria are met and your will is witnessed by at least two credible witnesses, the fact that it is handwritten does not invalidate it.
The witnesses to your holographic will must be adults. If either of them is a beneficiary in your will, you must have three witnesses. If both your initial witnesses are beneficiaries, you will need four. Alternatively, the beneficiary witnesses can waive anything you bequeath them that is more than they would have received if you had died without a will. Your witnesses also cannot be married to anyone who is a beneficiary in your will or the same rule regarding bequests applies to their spouses. Your will is likely to be acceptable if your witnesses meet these rules and sign your handwritten will.
In addition to having the proper witnesses, make sure you sign your handwritten will, as well. Presumably, you are able to do so if you have handwritten the whole will, but if for some reason you are incapable of signing your name at the end, Illinois allows someone else to do it for you as long as it is done in your presence and in the presence of your witnesses.
Holographic wills not signed by witnesses are accepted in Illinois if they were made in another state that does not require witness signatures. Wills that meet the requirements of the state where they were made are generally accepted elsewhere. A will is also valid in Illinois if it is made in another country but meets all the terms of the Uniform International Wills Act.