Angiotensin II is a very potent chemical that causes the muscles surrounding blood vessels to contract, thereby narrowing the vessels. The narrowing of the vessels increases the pressure within the vessels causing high blood pressure (hypertension). Angiotensin II is formed from angiotensin I in the blood by the enzyme angiotensin converting enzyme (ACE). ACE inhibitors are medications that slow (inhibit) the activity of the enzyme ACE, which decreases the production of angiotensin II. As a result, the blood vessels enlarge or dilate, and blood pressure is reduced. This lower blood pressure makes it easier for the heart to pump blood and can improve the function of a failing heart. In addition, the progression of kidney disease due to high blood pressure or diabetes is slowed. ACE inhibitors are used for controlling blood pressure, treating heart failure, preventing strokes, and preventing kidney damage in people with hypertension or diabetes. They also improve survival after heart attacks. Because they prevent early death resulting from hypertension, heart failure or heart attacks, ACE inhibitors are one of the most important group of drugs. This book presents important research in this field.