  Singletons are those unitary irreducible modules of the Poincare or (anti) de
Sitter group that can be lifted to unitary modules of the conformal group.
Higher-spin algebras are the corresponding realizations of the universal
enveloping algebra of the conformal algebra on these modules. These objects
appear in a wide variety of areas of theoretical physics: AdS/CFT
correspondence, electric-magnetic duality, higher-spin multiplets,
infinite-component Majorana equations, higher-derivative symmetries, etc.
Singletons and higher-spin algebras are reviewed through a list of their many
equivalent definitions in order to approach them from various perspectives. The
focus of this introduction is on the symmetries of a singleton: its maximal
algebra and the manifest realization thereof.
