Clinical inertia is defined as lack of treatment intensification in a patient not at evidence-based goals for care. Clinical inertia is a major factor that contributes to inadequate chronic disease care in patients with diabetes mellitus, hypertension, dyslipidemias, depression, coronary heart disease, and other conditions. Recent work suggests that clinical inertia related to the management of diabetes, hypertension, and lipid disorders may contribute to up to 80 percent of heart attacks and strokes. Clinical inertia is, therefore, a leading cause of potentially preventable adverse events, disability, death, and excess medical care costs. This paper addresses three specific objectives: (1) to present a conceptual model of clinical inertia that takes into account recent developments in human factors research, cognitive science, and organizational behavior; (2) to operationally define clinical inertia and propose simple clinical protocols that can be used to identify and map its incidence across populations of patients and physicians; and (3) to propose future research to reduce clinical inertia by specifically targeting the root causes of the problem. Ultimately, a better understanding of clinical inertia and the development of specific interventions to reduce it may be a productive strategy to reduce passive errors that contribute to hundreds of thousands of adverse events and tens of thousands of premature deaths annually in the United States.