Monitored Natural Attenuation (MNA) is unique among remedial technologies in relying entirely on natural processes to achieve site-specific objectives. Site characterization is essential to provide site-specific data and interpretations for the decision-making process (i.e., to determine if site remedial goals can be met with MNA in appropriate remedial time frames), and to provide site-specific data and interpretations to design a performance monitoring system (i.e., to determine the necessary monitoring parameters, locations, and frequency for monitoring). This publication provides a framework for site characterization in the context of MNA and is intended primarily for project managers to use during the planning, tasking, implementation, and/ or review of site characterization where MNA may be considered as a potential remedial technology. This document presents a broad overview of technical issues including development of a conceptual site model, characterization variables, sampling locations and frequencies, problematic issues encountered at MNA sites and approaches to overcome them, and the interpretations related to the MNA decision-making process.