In this talk I will discuss superconformal theories in four dimensions obtained wrapping M5 branes on a Riemann surface. I will first review N=2 class S theories introduced some years ago by Gaiotto and then describe a generalization to the N=1 case. For this class of theories it is possible to write down a spectral curve encoding the properties of the chiral ring and I propose a method to determine from the spectral curve the scaling dimension of chiral operators in the SCFT. This proposal reduces to the correct prescription in the special case of N=2 theories. I will provide several consistency checks and apply this method to study some new superconformal theories.