Specification of the characteristics unique to primary care, as distinguished from secondary and tertiary care, has been difficult. Descriptions based upon the nature of problems actually seen by practitioners or those based upon the way in which patients come for care do not adequately distinguish primary care from nonprimary care. Definitions of primary care have stressed its first-contact aspects, coordinating features, comprehensiveness, and longitudinality. While these phenomena are adequate as gross descriptors, the inability to quantify them reduces their usefulness to planners and evaluators. Offered as a solution to this problem is a model which permits these descriptors to be defined as specific interrelationships among separate aspects of the structure (accessibility, range of services, identification of the eligible population, and continuity), process (utilization and problem recognition), and outcome of care.