We consider the two membranes problem for different operators, possibly nonlocal. We discuss the Holder continuity of the solutions and a viscosity solution approach to this problem. In particular we explain how to obtain $C^{1,\gamma}$ regularity of solutions, provided that the orders of the two operators are different. In the special case when one operator coincides with the fractional Laplacian, one obtains the optimal regularity and a characterization of the free boundary. Based on joint works with L. Caffarelli and O. Savin.