Renin plays a significant role in normal and pathophysiologic states. Among the various factors affecting renin secretion are the afferent arteriole acting as a baroreceptor, the macula densa as a sensor of sodium delivery to the distal tubule, and the adrenergic nervous system. There is contrasting evidence for a potential role of intrarenal renin in autoregulating the renal blood flow and glomerular filtration rate. Levels of plasma renin vary with the clinical setting. Emphasis is placed on the role of renin in controlling effective blood volume through its effect on vascular tone and volume by release of angiotensin and aldosterone.