  A short account of the physics of electrical discharges in gases is given in
view of its historical evolution and application to planetary atmospheres. As
such it serves as an introduction to the articles on particular aspects of
electric discharges contained in this book, in particular in the chapters on
lightning and the violent discharges which in the recent two decades have been
observed to take place in Earth's upper atmosphere. In addition of briefly
reviewing the early history of gas discharge physics we discuss the main
parameters affecting violent atmospheric discharges like collision frequency,
mean free path and critical electric field strength. Any discharge current in
the atmosphere is clearly carried only by electrons. Above the lower bound of
the mesosphere the electrons must be considered magnetized with the
conductivity becoming a tensor. Moreover, the collisional mean free path in the
upper atmosphere becomes relatively large which lowers the critical electric
field there and more easily enables discharges than at lower altitudes. Finally
we briefly mention the relation of such discharges as sources for wave
emission.
