The classical linear Calderón-Zygmund theory deals with optimal integrability properties of solutions to linear elliptic and parabolic equations. Crucial tools in this setting are the representation formulas via fundamental solutions and singular integrals. When passing to nonlinear equations such tools are obviously not available. Nevertheless in the last years a set of different methods has been developed in order to build a parallel theory that in fact largely intersects with nonlinear potential theory. I'll try to give an overview of the subject.