Laws governing wills vary from state to state and are usually located in a state’s statutes—in its estates code, for example. Generally, a will is a legal declaration of a person's intention or wish regarding the disposition of his property, the guardianship of his children, and the administration of his estate upon his death. It is said to be a means of transferring title to property. Any writing or declaration by which a person intends to dispose of his property or estate upon his death is a will, and if done as required by law, is entitled to probate as a will. A will vests no present interest in the property it intends to distribute—it is only effective upon the death of the person making the will (the testator). And a will is ambulatory—meaning the testator can change or revoke it during his lifetime.

A will may (1) direct how the testator's property should be disposed of upon his death; (2) direct how not to dispose of the testator's property, such as by expressly disinheriting a child or other heir; (3) revoke a prior will; and (4) appoint an executor of the testator's estate, and a guardian for the testator's children.

Some common types of wills include:

Holographic wills—a holographic will is a will that is written entirely in the testator's handwriting. A will intended by the testator as holographic will be enforced even though it contains words not in the handwriting of the testator, if such other words are not necessary to complete the will and do not affect its meaning. For example, an acknowledgment of a notary public and the notary public's signature are surplusage not necessary to complete the will and do not affect its meaning. A holographic will is not required to be attested by subscribing (signing) witnesses.

Attested wills—an attested will is a will that is attested to (declaring or swearing to the truth of) by two or more credible witnesses who are at least 14 years of age, and who sign their names to the will in their own handwriting in the testator's presence. An attested will can be handwritten by the testator, but usually is not.

Codicils—a codicil is a supplement to a previous will that adds to, modifies, or revokes one or more provisions of the earlier will.

In Missouri, wills are governed by state statutes found in the Missouri Revised Statutes, specifically in the Probate Code. A will in Missouri is a legal document that expresses a person's wishes regarding the distribution of their property and the care of any minor children upon their death. To be valid, a will must comply with the legal requirements set forth by Missouri law. The testator, the person creating the will, must be of sound mind and at least 18 years old. Missouri recognizes both holographic and attested wills. A holographic will, which is entirely handwritten by the testator, does not require witness signatures to be valid, but it must be clear that the document is intended to serve as the testator's will. An attested will, on the other hand, must be signed by at least two credible witnesses who are at least 18 years old, and these witnesses must sign in the presence of the testator. Codicils, which are amendments to a will, must meet the same requirements as the original will to be legally binding. It's important to note that while a will can direct the disposition of property, disinherit individuals, revoke prior wills, and appoint an executor or guardian, it only becomes effective upon the testator's death and can be changed or revoked at any time during the testator's lifetime.