  We describe a new correspondence between four-dimensional conformal field
theories with extended supersymmetry and two-dimensional chiral algebras. The
meromorphic correlators of the chiral algebra compute correlators in a
protected sector of the four-dimensional theory. Infinite chiral symmetry has
far-reaching consequences for the spectral data, correlation functions, and
central charges of any four-dimensional theory with ${\mathcal N}=2$
superconformal symmetry.
