Good coordination of care in the ambulatory setting has the potential to reduce unnecessary or duplicative use of health services, prevent hospitalizations for ambulatory care–sensitive conditions, improve patient safety, and potentially reduce costs. Unfortunately, coordination failures are common across the health care system. Using performance measures can drive practice improvement, particularly if reimbursement aligns with measurement. However, there are few well-developed, standardized measures of care coordination. This study sought to develop electronic health record–based measures to assess the quality of coordination during the primary care physician-to-specialist referral process, one of the most common transitions across providers in health care. Using input from interviews with primary care physicians and experts, the authors developed a core set of five electronic measures for use in primary care and specialist settings. Through a preliminary evaluation, they determined that the measures are valid with practicing physicians and two are ready for implementation.