This talk is about the 4D/2D duality by Beem et al., a new connection between quantum field theory and representation theory discovered rather recently. It associates an algebraic object called VOA to any 4-dimensional superconformal field theory with N=2 supersymmetry, and the resulting VOA is conjecturally a complete invariant of the 4-dimensional theory. I will talk about the rich structure of the representation theory of the VOAs arising from 4-dimensional theory by this duality.