Superluminal motion in extragalactic jets is successfully explained assuming a relativistic flow of material in a direction close to the observer's line of sight. In rectilinear jets, the apparent velocity measured by the observer (βapp ) is related with the intrinsic velocity of the moving component (β) and the angle between the jet direction and the observer's line of sight (θ) through the equation (eg. Rees 1966, Nature 211, p.468):