The American Meteorological Society, under joint sponsorship from the U.S. Environmental Protection Agency, the Atmospheric Environment Service, Canada and the Ontario Ministry of the Environment, Canada, convened a workshop in September 1984 to address an aspect of long-range transport modeling that has become a topic of considerable concern, that is, the quantification of uncertainty in long-range transport model predictions and the implications of these uncertainties on formulations of control policy. A series of draft chapters provided a focus for discussion by 38 invited national and international experts in the areas of policy perspectives, methods for sensitivity and uncertainty analysis, and sources of errors and uncertainty in Eulerian, Lagrangian, and stochastic long-range transport models.