  For explaining electrical breakdown, field electron emission (FE) is a
mechanism of interest. In the period 2006 to 2010 there were significant
developments in basic FE theory, but these have not yet fully entered general
thinking in technological FE areas, which are often still based on 1960s
thinking or (in some contexts) 1920s thinking about FE theory. This paper
outlines the history of FE theory and provides an overview of modern
developments and of some related topics, in so far as these affect the
interpretation of experiments and the explanation of physical phenomena. The
paper concentrates on principles, with references given where details can be
found. Some suggestions are made about moving to the use of "21st-Century" FE
theory. In addition, an error in Feynman's treatment of the electrostatics of
pointed conductors is displayed, and it is found that Zener tunneling is
implausible as a primary cause of vacuum breakdown from a CuO overlayer.
