  Any four-dimensional $\mathcal{N} \geqslant 2$ superconformal field theory
possess a protected subsector isomorphic to a two-dimensional chiral algebra
\cite{Beem:2013sza}. The goal of these lectures is to provide an introduction
to the subject, covering the construction of the chiral algebras, the
consequences for four-dimensional physics, as well as a brief summary of recent
progress. This is the writeup of the lectures given at the Winter School "YRISW
2020" to appear in a special issue of JPhysA.
