EPA is developing a new mobile source emissions model that is comprehensive in source category, pollutant and analysis scale. A primary impetus for this effort is the National Research Council’s review of EPA’s mobile source modeling program, published in 2000, which recommended a) the development of a modeling system more capable of supporting smaller-scale analyses; b) improved characterization of emissions from high-emitting vehicles, heavy-duty vehicles, and off-road sources; c) improved characterization of particulate matter and toxic emissions; d) improved model evaluation and uncertainty assessments; and e) a long-term planning effort coordinated with other governmental entities engaged in emissions modeling. A cross-agency team representing OTAQ, OAQPS, ORD and Regional offices published an initial proposal to address these recommendations in April 2001. Since then, work has progressed on developing a conceptual design and software framework for the new modeling system. This design has been informed by recent advances in subregional emissions modeling and considers how the new mobile source emissions model should interact with new generation transportation and air quality models. Other important considerations of the design are how to capitalize on methods for improved characterization of in-use emissions, such as on-board emissions measurement, and how to incorporate model validation and uncertainty evaluation into the model structure. We present the latest information on a design and implementation plan for the model.