The functionality of dairy proteins is further enhanced upon liberation of bioactive peptides by proteolysis caused by naturally occurring enzymes in milk, and those in bacteria from starter cultures and probiotics. Among various bioactive peptides, antihypertensive peptides have been studied extensively. These peptides inhibit angiotensin I-converting enzyme (ACE), the key enzyme responsible for the regulation of blood pressure via the renin-angiotensin system. ACE converts angiotensin I to angiotensin II, a potent vasoconstrictor; ACE also hydrolyzes and inactivates bradykinin, a potent vasodilator. Therefore, excessive activity of ACE leads to an increased rate of vasoconstriction and development of high blood pressure. Inhibitory peptides block the ACE-mediated production of angiotensin II, and the reduction in ACE activity results in enhanced levels of bradykinin, resulting in an overall antihypertensive effect. Various food-derived peptides possess ACE inhibitory (ACE-I) properties. Caseins are important sources of those peptides. Biological significance of ACE-I peptides, their impact on human health and incorporation in functional foods have been the subject of intense research. Some antihypertensive peptides present in sour milk resist in vivo degradation and exert antihypertensive activity through the inhibition of ACE in the aorta. Most of the studies on the ACE-I production by fermentation are performed with selected strains of lactic acid bacteria mainly L. helveticus and direct hydrolysis of milk proteins with purified enzymes. A variety of naturally formed bioactive peptides have been found in fermented dairy products such as yogurt, sour milk, and cheese. These peptides are known to have multifunctional properties including immunostimulatory, opioid, hypotensive, antithrombotic, and antimicrobial activities. Several commercial products with highly proteolytic strains of L. helveticus have been developed and marketed to possess hypotensive activity including Calpis sour milk, prepared by fermenting milk using mixed culture containing L. helveticus CM4 (CP790) and Saccharomyces cerevisiae, which are responsible for the release of 2 tripeptides, Val-Pro-Pro and Ile-Pro-Pro, and Evolus (Valio Ltd., Finland), which is produced by highly proteolytic L. helveticus LBK-16H.