Intraaortic balloon pump counterpulsation is the most common form of mechanical circulatory support used in patients with myocardial ischemia and cardiogenic shock. The physiologic principles of counterpulsation include diastolic augmentation of aortic pressure and systolic reduction of left ventricular afterload, resulting in hemodynamic benefits through increased coronary perfusion pressure and improved myocardial oxygen balance in patients with myocardial ischemia. Major trials have failed to conclusively demonstrate improvements in morbidity and mortality with counterpulsation therapy for patients with acute myocardial infarction (MI), cardiogenic shock, and/or severe coronary artery disease undergoing revascularization therapy, and the debate over its applications continues. Part I of this review focuses on the history of the development of counterpulsation, technical considerations, and complications associated with its use, its physiologic effects, and evidence for its use in myocardial ischemia and cardiogenic shock.