In the law, a will or testament is a documentary instrument by which a person (the testator) regulates the rights of others over the testator's property or family after their death. For the devolution of property not disposed of by will, see inheritance and intestacy. In the strictest sense, "will" is a general term, while "testament" applies only to dispositions of personalty (this distinction is seldom observed).

The conception of freedom of disposition by will, familiar as it is in modern England and the United States, both generally considered common law systems, is by no means universal. In fact, complete freedom is the exception rather than the rule. Civil law systems often put some restrictions on the possibilities of disposal.

Some advocates for gays and lesbians have pointed to the inheritance rights of spouses as desirable for same-sex couples as well, through same-sex marriage or civil unions.

Some wills have unusual wishes the deceases wishes to be carried out after his death. For instance, Charles Vance Millar's will was notorious for presenting a cash prize for the family who births the greatest number of children in Toronto during the ten year period after his death which produced an unusual competition called the Stork Derby.

Though most people are aware that they need a will, many Americans (as high as 66 percent according to Consumer Reports) don't have one. Among the famous people who died without a valid will are Abraham Lincoln, Andrew Johnson, Ulysses S. Grant, Howard Hughes, Martin Luther King, Jr., and Pablo Picasso.

See also Will (law): legal history