The realization during the last decade that many water supplies worldwide are affected by arsenic at concentrations above acceptable health levels has motivated research directed at understanding the occurrence, distribution, and mobilization of arsenic in the environment, and stimulated the development of new, cost-effective treatment technologies. Arsenic accumulation and migration is closely tied to its chemical speciation, which is often controlled by a complex combination of abiotic and biotic processes coupled with physical transport. This symposium volume focuses on bridging the gap between different approaches and scales of investigation and on fostering a more unified understanding of arsenic occurrence and behavior, including geological, geochemical, hydrological, microbiological, ecological, and engineering aspects of arsenic-related research. Papers in this volume highlight a variety of new research directed at understanding the sources, distribution, and mobilization of arsenic in the environment. It includes recent efforts in the development of cost-effective treatment technologies and in approaches to natural attenuation and accelerated remediation methods. These topics are thematically organized into three sections in the volume, the first focusing on laboratory studies and theoretical modeling, the second on arsenic behavior and cycling in a range of field settings, and the third on studies associated with treatment and remediation technologies and methods. This book is targeted at scientists and engineers (chemists, geochemists, geologists, hydrogeologists, microbiologists, molecular biologists, chemical engineers, environmental engineers) as well as graduate students who are engaged in research on arsenic in both natural and engineered aqueous systems, and on the development of methods for arsenic treatment and removal.