[en] The physical processes underlying the Goldreich-Keeley instability are examined. This is a radiative fully-electromagnetic instability for a single plasma, requiring no relative streaming of plasmas. It is argued that the very general physical requirements for instability and the difficulty of quenching it by a spread in particle velocities implies that it is widespread in astrophysical environments, particularly pulsars. For reasonable pulsar parameters this radiative instability grows much faster than the beam-plasma modes usually invoked to explain charge bunching, and thus should dominate the characteristics of the observed radiation. (author)