Argyres-Douglas (AD) theories are 4d N=2 superconformal field theories without useful Lagrangian descriptions. Therefore their superconformal indices cannot be evaluated by supersymmetric localization. In this talk, I will discuss our conjectural expression for the superconformal index of AD theories given in terms of 2d q-deformed Yang-Mills theory. Our conjecture is based on the S^1 x S^3 version of the AGT relation, and is perfectly consistent with the Higgs branch chiral rings, 2d chiral algebras, RG-flows, and the 3d reduction of AD theories.