Over the past 15 years, endosaccular platinum coil therapy for intracranial aneurysms has evolved from clinical pilot studies of investigational devices to common clinical practice. The mechanism by which these coils reduce the risk of aneurysm rupture-the primary goal of intracranial aneurysm treatment-is the focus of this review. Both histological mechanisms of scar formation and hemodynamic mechanisms of flow diversion may be involved. We will first review aneurysm epidemiology to provide the context and rationale for therapy for patients harboring intracranial aneurysms. Next, we will review the data for and theories of the pathophysiology of aneurysm formation, growth, and rupture, particularly as they relate to endovascular coil therapy. Histological and hemodynamic studies of coiled aneurysms in animals and humans will be reviewed. Finally, we will discuss emerging coil-based therapies, such as bioactive polymer coatings for platinum coils and the adjunctive use of stents.