One of the key open questions is the phase structure of QCD and the nature of the transition between ordinary hadronic matter and the deconfined state of quark-gluon plasma taking place at extreme temperatures in excess of trillions of Kelvin. This talk will cover how properties of strongly interacting matter can be studied in the laboratory, in particular the search for the QCD critical point at finite baryon number density with event-by-event fluctuations in heavy-ion collisions. I will discuss the dynamical description of proton number cumulants in heavy-ion collisions based on relativistic hydrodynamics and the resulting constraints on the QCD critical point coming from recent data from Au-Au collisions at the Relativistic Heavy Ion Collider in Brookhaven.