A short review of photoemission studies of the early actinides is given. Emphasis is put on the gradual evolution of the actinide 5f states from bonding (delocalization) to nonbonding (localization) behavior. Localization is depicted by the withdrawal of the f states from screening of the photohole. Valence band and core level photoemission will be discussed. In the valence band, the transition from band like to localized states is seen in the appearance of multiplet peaks. Depending of the degree of f-localization, two different final states are reached (well and poorly screened), each with characteristic multiplet peaks. Core level spectra also show two different final states, depending of f-localization. Using photoemission data, the 5f transition to localization will be discussed for the pure elements. The effect on f-localization of dilution, that is, isolation in a matrix, confinement to reduced dimensions (surface layers), and chemical bonding with ligand atoms will be discussed. 5f Participation in ionic bonding and its consequences on the chemical reactions of actinide surfaces will then be addressed. 5f States are responsible for the multiple valence of actinides and are of high technological relevance. We will compare the oxidation behavior of U, Np, and Pu and focus on unusual intermediate and the highest oxidation states, which are all related to the number and the degree of localization of the 5f electrons. We then discuss the reduction of high oxidation states by reaction with hydrogen and the photocatalytic reduction by water.