Heart disease is the most common cause of death in the world. Most of our understanding of heart disease has involved the large heart arteries (epicardial arteries); however, disease of the very small heart arteries (coronary microvasculature) likely precedes the development of epicardial disease and represents the "base of the iceberg" of cardiovascular disease. Yet, we do not understand how dysfunctional microvasculature leads to reduced blood flow, symptoms and adverse outcomes.