The chronobiology of blood pressure is a useful tool for examining serial data to quantify the dynamics of variability. The implications and applications for diagnosis and therapy are discussed, with clinical examples. The chronobiological patterns of hypertension are described. Chronobiology can make a real contribution to the prevention of disease. Chronobiology helps diagnose circadian hyper-amplitude-tension (CHAT), a condition in which excessive circadian BP amplitude precedes chronic established hypertension.