Coded health care data from patients’ health records are used in epidemiological research, especially on incidence or prevalence of disease; for drug safety monitoring or long-term cohort tracking; and to inform policy making. This article briefly summarizes the evolution of internationally recognized coding ontologies and nomenclature and describes applications of coded electronic health record (EHR) data in day-to-day health care operations, research, auditing, and policy development. This article also illuminates how errors can occur when EHR information is coded, considers errors’ consequences, and suggests strategies for mitigating errors and improving overall use of coded EHR data.