Site-specific risk assessment is the process of evaluating whether or not a site poses a risk to human health or the environment. The purpose of all hazardous waste site remediation is ultimately to render a site safe for human or ecological populations. Consequently, risk assessment, as the process used to measure the effectiveness of the remediation process, is critical to all hazardous waste-site work. Risk assessment at hazardous waste sites involves the use of standard approaches and assumptions in a reasoned, common sense manner. The purpose of this book is to provide practical guidance to people wishing to learn about, conduct, or use risk assessment procedures in evaluating hazardous waste sites. Critical issues, standard formulas and assumptions, and guidance on characterizing risk results in a useable manner are presented. The use of risk assessment as a key tool in selecting appropriate remedial options at hazardous waste sites is also described. Most attention is given to human health risks associated with exposure to toxic chemicals, but descriptions of the strategies used to estimate radiation health risks and to evaluate risks to the environment are also provided. Although most commonly applied to hazardous waste site remediation, the procedures outlined in this book are generally applicable to any situation involving a potential for health risks to an exposed population. This book provides guidance on the mechanics of risk assessment preparation and illustrates these approaches with examples. However, the focus of the book is on the subjective nature of risk assessment, the art rather than the science. The actual risk (i.e., the right answer) can never be known. Consequently, while it is valuable to attempt to obtain the best numeric solution, reasonableness and the application of common sense are equally important. The book therefore devotes a substantial amount of space to issues of uncertainty that are inherent in risk assessment, and the need to address this uncertainty.