STOMP (Subsurface Transport Over Multiple Phases) is a continuum-scale simulator developed by Pacific Northwest National Laboratory. STOMP is used to simulate subsurface flow and transport and complements other analytical capabilities developed by PNNL's Hydrology Group. The simulator was specifically designed to provide scientists and engineers from varied disciplines with multidimensional analysis capabilities for modeling subsurface flow and transport phenomena. STOMP's target capabilities were guided by proposed or applied remediation activities at sites contaminated with volatile organic compounds and/or radioactive material. Developed with the support of the U.S. Department of Energy, Office of Environmental Restoration and Waste Management, the simulator's modeling capabilities address a variety of subsurface environments, including nonisothermal conditions, fractured media, multiple-phase systems, nonwetting fluid entrapment, soil freezing conditions, nonaqueous phase liquids, first-order chemical reactions, radioactive decay, solute transport, dense brines, nonequilibrium dissolution, and surfactant-enhanced dissolution and mobilization of organics. Available at: https://stomp.pnnl.gov/