This thesis represents the first in-depth ab initio (“first-principles”) study of many-body intricacies of photoemission processes for applications in next-generation high-brightness photoelectron emitters. Here, we focus on studying the Mean Transverse Energy (MTE), the fundamental parameter that limits the brightness, and thus the performance of a photoelectron emitter. We develop the first ab initio photoemission framework capable of calculating the MTE that includes full treatment of the relevant many-body processes, including two-body direct one-photon photoexcitation, three-body coherent electron-photon-phonon scattering, and three-body coherent two-photon photoexcitation. We then use this framework to explain the experimentally observed MTEs and predict the performance of various photocathode materials. Moreover, this thesis also includes a study on mechanical stability of the crystal structures of alkali antimonide photocathodes and clarifies a confusion in the current literature regarding the stable crystal structures of these materials. Finally, we also present a new ab initio framework that ultimately allows studies of photoemission processes in photocathodes with complex surfaces, which are of current and future interest because such complex photocathodes yield better operational lifetimes.