  This is the 11th article in the collection of reviews "Exact results in N=2
supersymmetric gauge theories", ed. J. Teschner. It describes an approach to
understanding the 4d/2d relations discovered by Alday, Gaiotto and Tachikawa by
establishing a triangle of relations between the zero mode quantum mechanics
obtained by localisation of class $\cal S$ theories, the quantum theory
obtained by quantisation of Hitchin moduli spaces, and conformal field theory.
