Under California law, some basic requirements must be met for a will to be valid. Anyone age 18 or over can make a will, provided that the person is mentally competent to do so. The California Probate Code makes it clear that a person is mentally competent to make a will as long as they understand the nature of what they are doing, understand what property they own and the extent of that property and know who their living spouse, parents, descendants and other potential beneficiaries are.
The person who makes a will is known as the "testator." For a will to be valid in California, it must generally be signed by the testator and by two witnesses. The witnesses must observe either the actual signing or the acknowledgment by the testator of his or her signature. A will can also be signed by another person, as long as that person did so in the testator's presence and at the testator's direction. A court appointed conservator can also sign a will on behalf of the testator, as long as they were appointed for the purpose of preparing the will.
As an alternative to these requirements, California law also recognizes what are known as "holographic wills." A holographic will is one in which the material terms of the will, as well as the signature, are in the testator's own handwriting. Because holographic wills are almost never prepared with the assistance of an attorney, they carry a high risk of errors that could result in frustration of the testator's wishes.
Source: Cal. Prob. Code §§ 6100, 6110-12, accessed Aug. 21, 2015