Electron sources for high-resolution applications such as microscopy and lithography are characterized by their brightness and energy spread rather than by their total emission current. In fact, most of the emitted current is cut out by apertures and only nanoamps or at most microamps actually reach the sample or wafer. This chapter reviews the physics and practical properties of the electron sources that are typically used in microscopy and lithography. It is an extension of topics addressed in  with special emphasis on application aspects and their emitter requirements. Recent developments are reviewed. The relations between the source properties and the resolution and throughput of these instruments is derived and discussed.