We discuss the physics underlying event-by-event fluctuations in relativistic
heavy ion collisions. We will argue that the fluctuations of the ratio of
positively over negatively charged particles may serve as a unique signature
for the Quark Gluon Plasma.Comment: Contribution to the Proceedings of the International Workshop XXX on
Gross Properties of Nuclei and Nuclear Excitations, Hirschegg, Jan. 2002 and
to the Proceedings of the Krakow Epiphany Conference, Krakow, Jan. 200
