  In this article we begin by reviewing the (Fang-)Fronsdal construction and
the non-local geometric equations with unconstrained gauge fields and
parameters built by Francia and the senior author from the higher-spin
curvatures of de Wit and Freedman. We then turn to the triplet structure of
totally symmetric tensors that emerges from free String Field Theory in the
$\alpha' \to 0$ limit and to its generalization to (A)dS backgrounds, and
conclude with a discussion of a simple local compensator form of the field
equations that displays the unconstrained gauge symmetry of the non-local
equations.
  Based on the lectures presented by A. Sagnotti at the First Solvay Workshop
on Higher-Spin Gauge Theories held in Brussels on May 12-14, 2004
