Just like there are formalities as to how the witnesses must witness a valid Will, there are requirements that govern who may act as witnesses.

Generally

California Probate Code Section 6112(a) states: Any person generally competent to be a witness may act as a witness to a will.. Although, it is not a strict requirement that the witness be 18 years or older, it is strongly recommended that only adults act as witnesses to avoid later challenges that a witness lacked competence due to age.

Interested Witnesses

An interested witness is someone who stands to receive something from the Will. A Will is not invalid just because one of the witnesses was an "interested witness." That being said, unless there are at least two other witnesses who are disinterested, there is a presumption that the interested witness obtained the bequest by duress, fraud, or undue influence.

California Probate Code Section 6112(c) states:

Unless there are at least two other subscribing witnesses to the will who are disinterested witnesses, the fact that the will makes a devise to a subscribing witness creates a presumption that the witness procured the devise by duress, menace, fraud, or undue influence. This presumption is a presumption affecting the burden of proof. This presumption does not apply where the witness is a person to whom the devise is made solely in a fiduciary capacity.

Unless the presumption is rebutted, the interested witness receives no more than what he or she would have received if the decedent died intestate.

California Probate Code Section 6112(d) states:

 If a devise made by the will to an interested witness fails because the presumption established by subdivision (c) applies to the devise and the witness fails to rebut the presumption, the interested witness shall take such proportion of the devise made to the witness in the will as does not exceed the share of the estate which would be distributed to the witness if the will were not established. Nothing in this subdivision affects the law that applies where it is established that the witness procured a devise by duress, menace, fraud, or undue influence.

Lawyer or Hospital Worker as Witnesses

Estate planning lawyers frequently act as witnesses to their client's Wills. Having a lawyer act as a witness may be especially helpful since there are generally good public records on where he or she is located should there be a need for them later on. Hospital employees are permitted to ac as witnesses; however, some medical institutions may disallow this.