The world of scanning electron microscopy seems to spin in such a way that every four years there is a dramatic step forward. Field emission sources (FEG), developed in the late 1960's, started life as a commercial disaster. In spite of the problems, certain manufacturers persisted with the first really user friendly FEG SEM reaching the market in the early 1980s. The FEG has been developed through the 1990's to be, without doubt, the source for SEU. But other advances in SEM performance have added to the instrument's performance, the most recent dramatic step being the introduction of semi-inlens imaging, a technique based upon 1980's dual detector imaging systems.