This book features papers presented at a workshop discussing current knowledge about the biological effects of low level exposures (BELLE). The book is designed to help establish a scientific base for future BELLE initiatives and is focused on the issue of the toxicological implications of biological adaptations. Hormesis is considered in a broad, conceptual manner, as well as at molecular and biochemical levels. Other topics covered include the effects of low levels of radiation on biological systems, how the liver adapts to genetic insults, biostatistical considerations when designing studies that address issues associated with biological responses to low doses of chemicals and radiation, and the issues that surround the interpretation of findings from such studies.