Thermal fluctuations can change the state of matter with increasing temperature e.g. when water boils to steam. Similarly, it is possible to tune the effective strength of quantum fluctuations demanded by the uncertainty principle---this happens when the ground state wave function of a large quantum system undergoes a qualitative change across a quantum phase transition. Such transitions have been observed in a variety of experiments. Some illustrative examples in transition metal compounds and trapped ultra-cold atoms will be presented. Right at the transition point, we obtain a novel "quantum critical" state of matter. Its description has intriguing connections to the quantum theory of black holes and the quark-gluon plasma. These fascinating subjects will be described by Professor Subir Sachdev, that is a world leader in their exploration