Angiotensin-converting enzyme inhibitors are an important treatment option for hypertension, especially when elevated blood pressure exists in the presence of diabetes mellitus, chronic kidney disease, or congestive heart failure. This article reviews some of the pathophysiologic mechanisms involved in patients with hypertension and these comorbidities and how they relate to the renin-angiotensin system (RAS). Inhibition of the RAS when utilized along with other antihypertensive medications has been particularly effective in hypertensive patients with type 2 diabetes, chronic kidney disease, and vascular disorders; consensus group guidelines have reflected this in their treatment recommendations. Clinical trial data demonstrate that the effectiveness of RAS blockers is enhanced by maximizing the daily dose and combining these medications with thiazide diuretics.