The event-by-event fluctuations in heavy ion collisions carry information about the thermodynamic properties of the hadronic system at the time of freeze-out. By studying these fluctuations as a function of varying control parameters, such as the collision energy, it is possible to learn much about the phase diagram of QCD. As a timely example, we stress the methods by which present experiments at the CERN SPS can locate the second order critical point at which a line of first order phase transitions ends. Those event-by-event signatures which are characteristic of freeze-out in the vicinity of the critical point will exhibit nonmonotonic dependence on control parameters. We focus on observables constructed from the multiplicity and transverse momenta of charged pions. We find good agreement between NA49 data and thermodynamic predictions for the noncritical fluctuations of such observables. We then analyze the effects due to the critical fluctuations of the sigma field. We estimate the size of these nonmonotonic effects which appear near the critical point, including restrictions imposed by finite size and finite time.