The National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (SNL) have developed WEC-Sim to provide the wave energy converter (WEC) design community with an open-source simulation tool. WEC-Sim models the system dynamics of WEC devices using multi- body dynamics methods and simulates hydrodynamic forces using coefficients predicted from potential flow models. In this paper we describe the methodology used in WEC-Sim and demonstrate the use of the code by simulating three WEC devices. Specifically, we model a two- body point absorber and two oscillating surge devices. For each device we describe how the WEC-Sim model was setup and present simulation results, such as predictions of device motions and power production. For verification and validation purposes, results are compared to corresponding results from other modeling tools and experimental data.