An international workshop on Conceptual Model Development for Subsurface Reactive Transport Modeling of Inorganic Contaminants, Radionuclides, and Nutrients was held April 20- 24, 2004 at La Posada de Albuquerque in Albuquerque, New Mexico. The workshop was organized and convened by the Federal Working Group on Subsurface Reactive Solute Transport Modeling and sponsored by the Federal Interagency Steering Committee on Multimedia Environmental Models (ISCMEM). The workshop themes were organized by processes: physical properties/coupling, sorption, precipitation/dissolution, and redox/microbiology. The workshop objectives were to 1) evaluate the state-of-the-art in conceptual model development and parameterization for reactive transport modeling as applied to soil, vadose zone, and groundwater systems and 2) prioritize research directions that would enhance the practical utility of reactive transport modeling. Plenary presentations and discussions provided an overview of critical modeling issues and needs, various agency applications, specifics of the four primary themes, and examples of complex applications of reactive transport models. Breakout discussion groups generated reports summarizing the motivation, current state-of-the-science, expectations, and recommendations for research in each of the four theme topics.