Periosteal new bone formation (PNBF) is the means by which appositional bone growth normally takes place on the surfaces of compact bone. Alterations in the periosteal microenvironment trigger complex interactions between osteoblasts and endothelial cells to promote PNBF. Physiologic processes like mechanical stress result in normal PNBF; but, a variety of pathologic processes result in excessive PNBF. The production of sufficient bone to be detectable by conventional radiography is a common feature of diverse etiologies, including infection; inflammation; prostaglandin E2 administration for ductal-dependent congenital heart disease; metabolic and hormonal abnormalities; neoplasms; fracture repair; systemic hypoxia; and hypertrophic osteoarthropathy. While the clinical settings and distribution of affected bone sites in these conditions are different, the histopathology of the PNBF is essentially identical; so, it seems logical that a common pathway might mediate them all. By combining the observations and insights gained from osseous research and studying the clinical pathology of these diverse conditions, we constructed a comprehensive pathway to explain PNBF. In doing so, it seems likely that Vascular Endothelial Growth Factor (VEGF) is the most likely common mediator of the pathways that lead to PNBF.