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 Montana, wills are governed by state statutes found in the Montana Code Annotated (MCA), particularly in Title 72, which pertains to estates, trusts, and fiduciary relationships. A will in Montana 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 requirements set forth by Montana law. A holographic will, which is entirely handwritten by the testator, is recognized in Montana as long as it is signed and the material provisions are in the handwriting of the testator (MCA 72-2-522). It does not require witness signatures. An attested will, on the other hand, must be signed by the testator and by at least two witnesses who are present during the signing and understand that the document is the testator's will (MCA 72-2-521). A codicil is a document that amends, rather than replaces, a previously executed will and must meet the same legal requirements as a will to be valid. Wills in Montana are ambulatory, meaning the testator can change or revoke them at any time before death, and they only take effect upon the testator's death. Additionally, Montana allows for the appointment of an executor to manage the estate and a guardian for any minor children within the will.