Abstract: Superconducting circuits are leading candidates for realizing quantum bits ("qubits"). To build a useful quantum computer, it is necessary to understand and mitigate energy relaxation in these qubits. Currently, energy relaxation is dominated by amorphous oxide layers residing at circuit interfaces. Here we study energy relaxation as a function of qubit frequency and time for various qubit geometries. We compare our experimental results with Monte Carlo simulations, and develop a model that describes energy relaxation due to defects residing at the substrate-air interface and along the Josephson junction leads.