Understanding the way valves work have moved on from the simple open and close movement. Their function is to counter the gravitational forces, lower ambulatory venous pressure and facilitate antegrade flow back towards the heart. Therefore, they play a critical role in the maintenance of a healthy venous system. Valve failure leads to retrograde leakage which is termed reflux. This can be tested using a variety of provocation manoeuvres which either invoke the calf muscle pump from below or increase venous pressures from above, like the Valsalva manoeuvre. Venous valves are almost invariably bicuspid and are situated within a valve sinus. Adjacent valves are off-set which suggests that they promote helical flow patterns in veins. The way they modulate flow is a growing area of research. Four phases of the valve cycle have been described.