The spent electron beam in an electron tube carries power which must be dissipated safely as heat. In gridded tubes, magnetrons and most CFAs the power is dissipated on the anode. In linear-beam tubes and gyrotrons the beam is allowed to expand to give a reduced power density on a separate collector electrode. In every case the surface temperature must be low enough to avoid physical damage or out-gassing of the collector surface. The collecting electrode may be cooled by conduction, air or liquid flow or liquid boiling. The power dissipated in the collector of a linear-beam tube can be reduced by using a multi-element depressed collector in which the collecting electrodes are held at negative potentials with respect to the body of the tube. In these collectors it is necessary to ensure that the performance is not reduced by the emission of secondary electrons. The design of collectors is discussed including the effect of the distribution of velocities in the spent electron beam.