Measurement of renin and its activity is increasingly used in the evaluation of circulatory function. This is primarily driven by a higher than previously realized prevalence of primary hyperaldosteronism. Renin measurement is also useful in evaluation of adrenal insufficiency, hypertension with hypokalemia, and assessment of suspected renin hypofunction. Renin measurement in the assessment of renovascular causes of hypertension has fallen out of favor due to poor predictive capacity in most situations. Measurement of renin typically occurs via two means. Plasma renin activity provides a measure of enzymatic performance by calculating the rate of angiotensin I formation by renin. Alternatively, renin can be directly measured by assessing the mass of renin in concentration. Physiologic conditions are important modifiers of renin activity. These include body posture, dietary salt intake, diurnal variation, age, race, renal disease, and confounding medications. As renin determination becomes more widely used in clinical practice, a solid understanding of assay limitations and the host of factors that influence renin activity will be required to properly interpret results.