Chlorinated solvents are altered by biotic and abiotic processes. Biotic transformation can include reductive dechlorination, cometabolism, and limited oxidation. Abiotic transformation is less well understood but may play a role at some sites. Transformations may be limited such that endpoints fall short of complete degradation of the solvent to innocuous compounds. Determination of which endpoints are reached, the processes of transformation, and the needed site data are critical for assessing and modeling transport, and deciding on monitored natural attenuation (MNA) as a remedy. This Issue Paper summarizes the biotic and abiotic transformations of several important chlorinated solvents. It briefly describes the factors that affect the transformation mechanisms, as well as the measurements necessary to distinguish among the mechanisms. It serves as a guide for developing an advanced ground-water transport model, with governing equations for simulating these processes in models. The primary audience is the EPA remedial project managers (RPMs). The Issue Paper is intended to provide RPMs with a basic understanding of the fundamentals and terminology of chlorinated solvent transformation in the context of MNA.