This article describes physical characteristics of the cesium telluride thin film as a photocathode material for an RF electron gun. The electron gun using the photocathode is also overviewed at the beginning of the article. The performance of photocathode in relation to a elemental ratio of cesium and telluride and to an electron affinity are discussed in detail referring new results obtained from recent studies in addition to accumulated knowledge, especially from the viewpoint of applications.