2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/53287We give a review of the extended conformal algebras, known as $W$ algebras, which contain currents of spins higher than 2 in addition to the energy-momentum tensor. These include the non-linear $W_N$ algebras; the linear $W_\infty$ and $W_{1+\infty}$ algebras; and their super-extensions. We discuss their applications to the construction of $W$-gravity and $W$-string theories.41 pages. This paper is based on a set of lectures given at the Trieste Summer School in High-Energy Physics, August 1991High Energy Physics - Theorytext