In clinical trials a study’s protocol has instructions or rules on who can or cannot participate in the study. These instructions, called eligibility criteria, describe characteristics that must be shared by all study participants. The criteria differ from study to study. Eligibility criteria include both inclusion and exclusion criteria and may be comprised of age, gender, medical history, and current health status, severity of the disease being studied, and concomitant medications. Eligibility criteria for treatment studies often require that patients have a particular indication, type of disease and stage of disease. There are many efforts to structure eligibility criteria for clinical trials, but none that have represented it in FHIR adequately to meet the business need. The structuring of eligibility criteria in FHIR will support several data exchange scenarios: a clinical trials registry that includes submission of eligibility criteria; supporting trial matching services; supporting the data exchange between hospital information systems and clinical trials management systems as well as trial planning efforts (including estimation of potential sample size at study sites).