Our book addresses the needs of practitioners, engineers, scientists, regulators, resource managers, planners, and others with a need to know about septic systems. It arose after discussions about the need for a text that integrated current understanding of the hydrologic, physical, chemical, and biological processes involved in the treatment of wastewater using soil. In our experience, people working with septic systems - ourselves included - have a fragmented understanding of what these systems are, how they function, how wastewater moves through soil, how and which pollutants are removed, and how these systems impact the environment and public health. The relevant information is scattered across disciplines, information sources and audiences. This book is an attempt to collect and integrate this information in one place, and provide a scientific framework for understanding soil-based wastewater treatment.