An Introduction to Free Higher-Spin Fields
  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 
N. Bouatta, G. Compere and A. Sagnotti