By formulating the six dimensional (0,2) superconformal field theory X[j] on a Riemann surface decorated with certain codimension two defects, a multitude of four dimensional N=2 supersymmetric field theories (usually called Class S theories) can be constructed. In the talk, I will describe various aspects of this construction for j=A,D,E. This will include, in particular, an exposition of the various partial descriptions of the codimension two defects that become available under dimensional reductions and the relationships between them. I will also describe a particular observable of this class of four dimensional theories, namely the partition function on the four sphere and its relationship to correlation functions in a class of two dimensional non-rational conformal field theories called Toda theories with a focus on the part of the 4d/2ddictionary that involves the Euler anomaly of a four SCFT of class S.