Edema is a common clinical problem, and the daily avoidance of edema depends critically on the lymphatic system, which clears leaked plasma proteins and fluid from the interstitial compartment. There is often confusion as to the difference between chronic edema and lymphedema. Lymphedema is by definition primarily a disease of impaired lymphatic drainage and lymph flow, and progress in lymphedema research, currently an increasingly active field, requires a clinically viable method for the quantitative assessment of lymph drainage rate in patients. Measurement of the rate of clearance of a new protein marker, radiolabelled human immunoglobulin, from skin, subcutis, and muscle provides a way of measuring human lymph flow quantitatively and is the only viable clinical method currently available. Considerable strides have been made over the last 5–10 years in evaluating the method and its pitfalls, including potential complications such as vascular clearance, peripheral lymphovenous communications and label dissociation. The review assesses critically, for the first time, the evidence relating to the method: its pitfalls; human lymph flow in various healthy and oedematous tissues; and how this is altered in hyperfiltration edemas, inflammation, vasoconstriction and various primary and secondary human lymphedemas.