  Observables 'are observed' whereas beables just 'are'. This gives beables
more scope in the cosmological and quantum domains. Both observables and
beables are entities that form 'brackets' with 'the constraints' that are
'equal to' zero. We explain how depending on circumstances, these could be,
e.g., Poisson, Dirac, commutator, histories, Schouten-Nijenhuis, double or
Nambu brackets, first-class, gauge, linear or effective constraints, and
strong, weak or weak-effective equalities. The Dirac-Bergmann distinction in
notions of gauge leads to further notions of observables or beables, and is
tied to a number of diffeomorphism-specific subtleties. Thus we cover a wide
range of notions of observables or beables that occur in classical and quantum
gravitational theories: Dirac, Kucha\v{r}, effective, Bergmann, histories,
multisymplectic, master, Nambu and bi-. Indeed this review covers a
representatively wide range of such theories: general relativity, loop quantum
gravity, histories theory, supergravity and M-theory.
