Abstract: The Common Data Element (CDE) Dictionary is an on-line vocabulary repository sponsored by the National Cancer Institute (NCI). CDEs, a unique language for cancer clinical trials, was developed to standardize the terminology utilized in clinical trials and to promote the uniform collection of study data. By providing a collection of approved terms, investigators in the field can expect that eligibility and data collection requirements for like items will be identical from one trial to the next in a given disease. In addition, CDEs should greatly facilitate data pooling and comparisons between studies, as analysts will have clear definitions and valid values for like data items. With the increasing use of electronic communication, a web- based Dictionary was created to collect and store these data terms in a publicly available site on the Internet. To create such a CDE Dictionary, consensus on the research and administrative terms was an essential prerequisite. A collaborative effort was initiated by the NCI with experts in a variety of oncology-related disciplines from the Clinical Trials Cooperative Groups along with other trialists to develop the terminology used to collect information specific for cancer clinical trials. Whenever available, terms and values associated with CDEs were chosen based on established standards. Each data element was also reviewed to see how it could best be represented in an electronic data collection application that was also used to provide an efficient search mechanism within the application. The Cancer Therapy Evaluation Program (CTEP) has implemented the use of CDEs in the development of Case Report Forms (CRFs) used for the collection of clinical trial data on national Phase 3 trials authored by the Clinical Trials Cooperative Groups. All CRFs are reviewed in CTEP to assess usage of the CDEs and staff is available to help those generating CRFs to develop CDEs addressing additional content that reflects the focus of the study. A change management process has been developed for both the content and the database architecture to provide a robust yet stable structure for the Dictionary. This process will also enhance the procedure for submission of new additions to the Dictionary and harmonize suggested changes to CDEs currently available. The goal is to maintain a common nomenclature across modalities and diseases whenever possible to provide consistency in the CDE terminology.