  Motivated by forthcoming experiments at RHIC and LHC, we study event-by-event
fluctuations in ultrarelativistic heavy-ion collisions in participant nucleon
as well as thermal models. The calculated physical observables, including
multiplicity, kaon to pion ratios, and transverse momenta agree well with
recent NA49 data at the SPS, and indicate that such studies do not yet reveal
the presence of new physics. Finally, we present a simple model of how a first
order phase transition can be signaled by very large fluctuations.
