The use of high efficiency GaAs transmission mode photocathodes in image tubes is an achievement which has been made possible thanks to the improvement of material technology and vacuum technology. As background into the description of the material technology, the device characteristics are used for the definition of the material criteria. Possible epitaxial structures and growth methods which have been studied for the preparation of the material are reviewed with emphasis on the GaAs/(Al,Ga)As/ transparent window type of structure. Recent progress in MOVPE shows that this technique is now capable of growing high quality GaAs(Al,Ga)As double heterostructures suitable for photocathode fabrication. The assessment of p-type GaAs active layers shows electron diffusion lengths of 5 to 7 μm for a doping level of 1 × 10 19 cm -3 with neglectible interface recombination. Reproduvibility of the results and further development of MOVPE for large scale growth of photocathode materials is discussed.