By higher-spin (HS) field one means generalizations of the electromagnetic potential or of the metric fluctuation that transform under arbitrary representations of the Lorentz group. Conceptual difficulties have long been identified in attempts to couple massless higher-spin modes in a Minkowski background. However, the key classic no-go theorems typically do not apply in the presence of infinite numbers of them. String Theory clearly leads the way to date, since its spectra involves a plethora of massive HS modes, however, a key question is whether String Theory itself is part of a more general structure for higher-spin interactions, and what role it possibly plays in it. For massive fields, one expects that an effective Lagrangian description be possible below the scale of their masses, and therefore in this Thesis String Theory is taken as a starting point to exhibit for the first time a number of couplings involving higher-spin modes. The novelty is here the explicit computation of tree-level scattering amplitudes for massive modes. The Weyl calculus allows to present the whole set of cubic and quartic string couplings and the resulting currents in a suggestive form.