  A contribution to the collection of reviews "Introduction to Higher Spin
Theory" edited by S. Fredenhagen, this introductory article is a pedagogical
account of higher-spin fields and their connections with String Theory. We
start with the motivations for and a brief historical overview of the subject.
We discuss the Wigner classifications of unitary irreducible
Poincar\'e-modules, write down covariant field equations for totally symmetric
massive and massless representations in flat space, and consider their
Lagrangian formulation. After an elementary exposition of the AdS unitary
representations, we review the key no-go and yes-go results concerning
higher-spin interactions, e.g., the Velo-Zwanziger acausality and its
string-theoretic resolution among others. The unfolded formalism, which
underlies Vasiliev's equations, is then introduced to reformulate the
flat-space Bargmann-Wigner equations and the AdS massive-scalar Klein-Gordon
equation, and to state the "central on-mass-shell theorem". These techniques
are used for deriving the unfolded form of the boundary-to-bulk propagator in
$AdS_4$, which in turn discloses the asymptotic symmetries of (supersymmetric)
higher-spin theories. The implications for string-higher-spin dualities
revealed by this analysis are then elaborated.
