Quantum chromodynamics (QCD) is the fundamental theory describing strong nuclear force. Despite its precise mathematical definition, much remains unknown regarding the properties of strongly interacting QCD matter in the non-perturbative regime. 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 laboratory, through ultrarelativistic collisions of heavy ions. A particular emphasis is put on the use of event-by-event fluctuations in the search for the QCD critical point at finite baryon number density. I will discuss the constraints on the QCD critical point coming from recent data from Au-Au collisions at the Relativistic Heavy Ion Collider in Brookhaven as well as future perspectives.