The physics of large scale jets in class I and class II extragalactic radio sources and quasars is discussed. Class I jets appear to be turbulent, transonic jets which entrain the interstellar medium. The related jet deceleration causes a slow surface brightness decline which is usually observed. Class II jets are supersonic and terminate in an advancing shock against the external medium. Both types of jet are initially light but the ratio of jet density to external density of class I jets increases owing to entrainment. It is quite plausible that quasar jets are hypersonic and light and this may solve problems of confinement. The velocities of class I jets are of the order of a few thousand kilometers per second. Class II and quasar jets may be at least mildly relativistic. However, it is not clear whether the velocities of large scale jets in powerful sources are close to the speed of light. Recent depolarization measurements provide an interesting focus for discussion of this question.