Monitored Natural Attenuation (MNA) remedies monitor the results of natural processes and utilize these data to predict future conditions in the aquifer. Performance monitoring to evaluate MNA effectiveness is a critical element for MNA remedies. Monitoring over time insures that the future behavior of the plume is consistent with past behavior and that the risk of exposure to the contaminants is managed. The trend of contaminant concentrations over time in a particular well can be used to forecast the future concentrations in that well and predict when concentrations will attain a selected concentration level. The purpose of this document is to present a simple, statistically based approach for evaluating the progress of natural attenuation from the data collected during site characterization and long term monitoring. The intended audience is technical professionals that actually perform the data analyses (i.e., hydrogeologists, engineers) as well as project managers who review those analyses and/or make decisions based on those analyses.