A reliable source of electrons to ‘illuminate’ the specimen is one of themost important parts of a TEM. Fortunately, electron sources are plentiful, but to get the best images and other signals out of our expensive microscope, we need to use the best available source. There are stringent requirements to produce the beam of electrons with the necessary properties and these are best met by only two types of source: thermionic and field-emission sources (or ‘guns’ as they are often called). Thermionic sources are (now rarely) tungsten filaments or (now commonly) lanthanum hexaboride (LaB
) crystals, and field emitters are fine tungsten needles. In this chapter we’ll first explain briefly the physics of the different electron-emission processes because then you’ll understand why we operate the sources in certain ways.