Electric thrusters are finding increasing usage worldwide in spacecraft applications. Significant progress has been made in recent years in the modeling and performance of thermionic hollow cathodes used in flight thrusters, such as Hall and ion thrusters, or other types of plasma sources, such as for technological plasmas. The recent progress is surveyed in this paper through the discussion of six areas: hollow cathode modeling and simulation, low-current hollow cathodes, high-current hollow cathodes, heaterless hollow cathodes, new thermionic insert materials, and plasma oscillations. This includes descriptions of hollow cathode designs capable of < 1 A to over 300 A, advances in electron emitter and heating/starting technologies, and modeling and simulation of the plasma properties, thermal behavior and instabilities in the discharge. Advances in the understanding and technology in these areas and challenges that still need to be addressed and solved are discussed.