Introduction Natural source zone depletion (NSZD) has emerged as an important concept within the realm of environmental remediation. NSZD is a term used to describe the collective, naturally occurring processes of dissolution, volatilization, and biodegradation that results in mass losses of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbon constituents from the subsurface. This document provides practical guidance on NSZD theory, application, measurement methods, and data interpretation. It is intended to be used by practitioners to help plan, design, and implement NSZD monitoring programs in support of petroleum hydrocarbon site remediation. This section of the document provides an introduction to the origin of the NSZD term, motivation, objectives, intended audience, and uses. To set the context for subsequent discussions, it also provides a broad overview on how measurements of NSZD can be used for decision making at remediation sites impacted by petroleum hydrocarbons.