Understanding of the factors affecting regurgitant flow through a heart valve and of the inherent limitations of the Doppler technique is needed to interpret correctly the information obtained during an ultrasound Doppler examination. This paper describes the flow conditions at the leaking valve and limitations of the Doppler technique which become important in the case of valve regurgitation. The flow conditions can be described in the following terms: contraction of the flow, core flow dimensions, friction, and intrusion and width of the jet flow. Contraction occurs at the entrance to the orifice and causes the width of the jet at the orifice to be smaller than the orifice itself. This contraction should be taken into account when calculating volume flow. The jet reaches a minimal area at the vena contracta where the flow velocity is close to that expected from the Bernoulli equation. The area of the vena contracta relative to the area of the hole can vary between 0.6 and 1.0; the lowest value is seen at a sharp-edged orifice and the highest value, at a hole with an ideally rounded inlet. Friction has a marginal role onflow velocity at the vena contracta. The velocity at the vena contracta persists in a region called the core flow region. This region has a length of 4–8 hole diameters. The total jet intrusion and the width of the jet are related to both the flow velocity at the hole and the diameter of the hole. Flow velocities lower than those expected from the Bernoulli relationship, when measured with the ultrasound Doppler technique, can be attributed to geometrical factors at the measurement and inherent limitations of the ultrasound Doppler technique.