Electronic medical records (EMRs) with sophisticated clinical decision support (CDS) functions are increasingly common in health systems that have affiliated clinical practice sites. Cluster-randomized trials (CRTs) of different approaches to CDS are facilitated by EMRs in these systems by: 1. enabling identification of patients and problem areas that might benefit from CDS; 2. providing rich clinical data for a priori balancing of practices on important characteristics before allocating practice clusters to intervention groups; and 3. providing platforms for developing different types of CDSs; for example, for patients or for providers. Course leaders will illustrate key points by highlighting an AHRQ-support CRT of CDS in diabetes across two organizations and 24 practice sites, and a small-group interactive task completed during the session will motivate the presentation.