Lymphedema is a common late toxicity of cancer therapy. This article describes the rationale and process utilized by the Lymphedema Working Group for the revision and expansion of the Common Toxicity Criteria version 2 (CTC v2.0) lymphedema criteria to produce the CTC v3.0 lymphedema criteria. Established clinician-based rating scales and quantitative instruments are reviewed in this article. None of the extant rating scales have been formally validated, nor has their reliability been assessed. Drawbacks of current scales were considered in formulating CTC v3.0 criteria. Most rely exclusively on volume to diagnose and grade lymphedema. This imposes significant clinical limitations, particularly in the assessment of toxicity in oncology clinical trials. Volume-based rating scales are of little value in rating the severity of bilateral limb and nonlimb edema. Problems with nonvolumetric staging systems (eg, CTC v2.0) include insufficient detail to permit useful discrimination of severity among the majority of lymphedema patients. Technologies for objectively quantifying lymphedema have been developed and validated. Although these are briefly reviewed, it is recognized that cost and access issues limit their widespread clinical utility and, as such, were not considered in developing the CTC v3.0 criteria. The CTC v3.0 lymphedema criteria adopted several innovations. Principle among these was the decision to generate separate criteria for volumetric increase, dermal changes, and subcutaneous fibrosis. We anticipate the use of the new CTC v3.0 lymphedema criteria to begin in mid-2003 for grading the key clinical features of this disorder in oncology clinical trials. The purpose of this article is to familiarize the reader with (1) background on the clinical features of lymphedema, (2) information on established lymphedema rating systems, (3) the consensus process and rationale of the Lymphedema Working Group, (4) the new CTC v3.0, and (5) quantitative techniques for assessment of lymphedema.