Despite a high level of support for the importance of clinical prevention, physician delivery of preventive services falls well below recommended levels. Competing demands faced by physicians during the medical encounter present a major barrier to the provision of specific preventive services to patients. These demands include acute care, patient requests, chronic illnesses, psychosocial problems, screening for asymptomatic disease, counseling for behavior change, other preventive services, and administration and management of care. This paper outlines how competing demands affect physician delivery of clinical preventive services and provides a model designed to help practicing physicians improve the delivery of preventive services. This model can be helpful in the planning of preventive interventions in primary care settings and can facilitate a better understanding of physician behavior.