Nonlocal elliptic equations have long been used by physicists and engineers to model diffusion processes involving jumps. Apart from several works from a probabilistic view, there had not been much development concerning their mathematical properties until the fundamental works of Caffarelli and Silvestre. Here we establish several results concerning the regularity of viscosity solutions to nonlocal elliptic equations. In particular, we show the existence of smooth solutions to two class of nonlocal fully nonlinear elliptic equations, an integrability estimate for the fractional order Hessian of solutions to nonlocal equations, as well as a theory of at solutions.