This chapter discusses modern developments in theoretical research of field emission from metals and semiconductors. Emitter shape approximation and methods of electrostatic potential and field strength calculation are considered. It also presents the theory of thermal-field emission (TFE) from metals. Energy distributions of field electrons and TFE electrons emitted from a metal are considered in terms of applications for field emission spectroscopy. Theoretical aspects of phenomena and processes on the emitter surface during field emission are discussed and various theories attempting to explain them are presented.