We discuss the physics underlying event-by-event fluctuations in relativistic
heavy ion collisions. We will emphasize how the fluctuations of particle ratios
can be utilized to explore the properties of the matter created in these
collisions. In particular, we will argue that the fluctutions of the ratio of
positively over negatively charged particles may serve as a unique signature
for the Quark Gluon Plasma.Comment: Proceedings Quark Matter 2001, Stony Brook, NY January 200
