A Symposium on applications and enhancements of the TOUGH family of numerical simulators was held May 12-14, 2003, at the Lawrence Berkeley National Laboratory (Berkeley Lab). As in previous TOUGH Workshops held at Berkeley Lab in 1990, 1995, and 1998, the TOUGH Symposium 2003 was organized as an open forum for information exchange among developers and users of the TOUGH family of nonisothermal multiphase flow simulators. Lectures were given and posters presented covering various application areas, including geothermal reservoir engineering, nuclear waste isolation, vadose zone and fracture flow hydrology, environmental remediation, mining engineering, carbon sequestration and hydrocarbon recovery, reactive transport, and others. Over 90 participants from 14 countries attended the three-day event, during which 60 technical papers were presented. The Symposium was concluded with a panel discussion and followed by a three-day TOUGH2 training course attended by 22 participants. This CD-ROM contains the papers (in PDF format) that were presented at the TOUGH Symposium 2003. The proceedings papers are also accessible through the Symposium Website at http://www esd.lbl.gov/TOUGHsymposium. The CD-ROM also includes papers from the 1995 and 1998 TOUGH Workshops. The papers compiled in these Proceedings demonstrate both the growing sophistication of the TOUGH codes, as well as the diversity of their applications. For example, there are papers on new developments in coupling thermal, hydrological, chemical, mechanical, and biological processes, advances in inverse modeling and optimization, and the increased use of high-performance computing methodologies. The use of TOUGH in classical application areas (such as nuclear waste isolation, geothermal reservoir engineering, and environmental remediation) has matured and now includes large site-scale models of considerable complexity. Furthermore, TOUGH appears to be flexible enough to address emerging scientific and engineering challenges, such as those related to geologic carbon sequestration or methane gas hydrate production problems. Innovative developments in pre- and postprocessing further enhance the utility of the code. We hope these proceedings will provide useful information for individuals and organizations interested in nonisothermal multiphase flow problems in permeable media. The Organizing Committee wishes to thank the session chairs, keynote speakers, presenters, and members of the panel for their contributions to the Symposium. The support fro m various agencies and offices for the development and application of the TOUGH codes is greatly appreciated. Given the widespread interest in TOUGH demonstrated during the Symposium, we are confident that TOUGH will continue to play an important role as both a research code and a simulation tool for practical applications in various areas of subsurface science and engineering. Berkeley, July 2003, The Organizing Committee, Stefan Finsterle, George J. Moridis, Curtis M. Oldenburg, Yu-Shu Wu