The design, construction, operation, and performance of a spin polarized electron source utilizing photoemission from negative electron affinity (NEA) GaAs are presented in detail. A polarization of 43±2% is produced using NEA GaAs (100). The polarization can be easily modulated without affecting other characteristics of the electron beam. The electron beam intensity depends on the intensity of the exciting radiation at 1.6 eV; beam currents of 20 &mgr;A/mW are obtained. The source is electron optically bright; the emittance phase space (energy‐area‐solid angle product) is 0.043 eV mm2sr. The light optics, electron optics, and cathode preparation including the GaAs cleaning and activation to NEA are discussed in depth. The origin of the spin polarization in the photoexcitation process is reviewed and new equations describing the depolarization of photoelectrons in the emission process are derived. Quantum yield and polarization measurements for both NEA and positive electron affinity surfaces are reported. The important considerations for interfacing he polarized electron source to an experiment are illustrated by its application to polarized low energy electron diffraction (PLEED). The advantages of this spin polarization modulated electron gun for PLEED are clearly demonstrated by sample PLEED results for W(100) and ferromagnetic Ni(110). A comparison with other polarized electron sources shows that the GaAs spin polarized electron source offers many advantages for a wide range of applications.