The physical principles underlying the design and performance of electron guns suitable for the generation of high‐density electron beams (beam current density of the order of 200 amp/cm2) are discussed. Initiation of the flow of a cylindrical electron beam focused by a uniform magnetic field is described, with emphasis on the design of the electron gun. The principles underlying the Pierce design procedure for space‐charge limited electron guns is treated, including the limitations and the modifications necessary when used for very high‐density beams. Means are suggested for partial compensation of the electric field distortion due to the anode aperture of the gun. The effects on the beam and resulting limitations in gun design due to thermal velocities of emission are described.The experimental techniques for study of gun and beam performance are described, together with some results of these studies. These techniques include detailed measurement of beam characteristics, such as current density and transverse velocity distributions, both with and without a magnetic field. These measurements illustrate a number of deviations of actual electron flow from that usually considered in the simple theories.