Time-changed Levy processes include the fractional Poisson process, and the scaling limit of a continuous time random walk. They are obtained by replacing the deterministic time variable by a positive non-decreasing random process. The use of time-changed processes in modeling often requires the knowledge of their second order properties such as the correlation function. This paper provides the explicit expression for the correlation function for time-changed Levy processes. The processes used to model random time include subordinators and inverse subordinators, and the time-changed Levy processes include limits of continuous time random walks. Several examples useful in applications are discussed.