This guide provides an introduction to calibrating transmission electron microscopes (TEM's) and scanning transmission electron microscopes (STEM's), as well as (S) TEM's - instruments that can perform in both modes. While not technically correct, all these instruments will be collectively referred to as TEM's for the rest of this Guide. As with most things, there is a wealth of interesting facts and insights to be discovered by exploring this topic further. More detailed explanations can be found in the classic monographs by Edington or in textbooks such as Williams and Carter.