A theory that denies that the specification of a physical system given by a state described by quantum mechanics is a complete specification. A successful hidden-variables theory has never been constructed. There are important considerations preventing the construction of a simple hidden-variables theory, notably Bell's theorem. The only type of hidden-variables theories that appears not to have been ruled out are nonlocal hidden-variables theories, i.e. theories in which hidden parameters can affect parts of the system in arbitrarily distant regions simultaneously. A hidden-variables theory that does not satisfy this definition is called a local hidden-variables theory.