Publisher Summary This chapter describes the importance of electrons as a hydrodynamical fluid. The pressure of the electron component of plasma can exert a significant effect upon the mechanical behavior of the plasma. Because in the absence of currents the action of this pressure cannot be distinguished from that of any other component by the morphology of the effects, identification of its importance depends on measurements of the electron temperature or on a demonstration of the occurrence of the effect at an epoch when no other cause could as yet be active. When electron pressure is predominant, however, it represents a more efficient use of energy than otherwise, providing what amounts to an internal electronic and magnetic field (EMF) for the acceleration of the heavy ionic component. A strong magnetic field can coerce plasma into hydrodynamical motion. An idealized treatment of the magnetic action, based on an infinitely thin current shell advancing inwards and driving a shock wave which rebounds to reverse the advance of the layer, gave a reasonable qualitative agreement. It is suggested that the phenomenon of the striated positive column of the low pressure glow discharge certainly has a hydrodynamical basis, and probably bears the same relation to the ionic sound waves that forced oscillations do to free oscillations as the observed wave speeds are in the same range.