An International Workshop on Uncertainty, Sensitivity, and Parameter Estimation for Multimedia Environmental Modeling was held August 1921, 2003, at the U.S. Nuclear Regulatory Commission Headquarters in Rockville, Maryland, USA. The workshop was organized and convened by the Federal Working Group on Uncertainty and Parameter Estimation, and sponsored by the Federal Interagency Steering Committee on Multimedia Environmental Models (ISCMEM). The workshop themes were parameter estimation, sensitivity analysis, and uncertainty analysis relevant to environmental modeling. The workshop objectives were to facilitate communication among U.S. Federal agencies conducting research on the workshop themes; obtain up-to-date information from invited technical experts; actively discuss the state-of-the-science in the workshop themes; and identify opportunities for pursuing new approaches. These objectives were met through the workshop presentations and discussions. The invited presenters focused on methods to identify, evaluate, and compare both existing and newly developed strategies and tools for parameter estimation, sensitivity and uncertainty analyses.