Flow is defined as the amount of fluid, ordinarily blood, passing per unit time (usually per minute). Perfusion is defined as the volume of blood flowing through a mass of tissue per unit time. The standard measure of perfusion is millilitres of fluid per second per 100 g of tissue (or 100 ml in volume) i.e. the fractional blood volume in an area divided by the mean transit time through the area, where the area is scaled to a 100 ml of tissue. The fractional vascular volume is an index of the physiological vascularity of a region and represents the proportion of tissue occupied by blood. Several ultrasound based techniques have been used in an attempt to quantify perfusion. Vessel dimensions of capillaries and arterioles are below the spatial resolution of even the latest ultrasound machines, and capillary flow is slower than that required to generate a Doppler signal which leads to underestimation of true perfusion. For all ultrasound-based techniques movement artefacts are probably the greatest obstacle to obtain quantitative information on perfusion. This review will discuss the technology and use of the ultrasound mode of three dimensional power Doppler, its benefits and limitations in estimating vascularity focusing on the field of obstetrics.