Historically, SEM developed as a high vacuum technique requiring sample chamber vacuum of 10-5 Torr or better. Energetic electrons scatter from any molecules they encounter, so their creation and transport from source to sample, through the focusing lenses of the electron column, requires high vacuum. This imposes a number of limitations on the kinds of samples that can be examined. Samples must tolerate the vacuum environment and the vacuum system must tolerate the sample. Generally, samples have to be solid, clean, dry, and not contain volatile components. Furthermore, since the vacuum insulates the sample from everything except the stub that supports it, non-conductive samples require a conductive pathway between the scanned region and ground to prevent the accumulation of charge deposited by the beam electrons.