Heavy ion collision experiments recreating the quark-gluon plasma that filled the nascent universe have established that it is a nearly perfect liquid that flows with such minimal dissipation that it cannot be seen as made of particles. String theory provides a powerful toolbox for studying matter with such properties. This book provides a comprehensive introduction to gauge/string duality and its applications to the study of the thermal and transport properties of quark-gluon plasma, the dynamics of how it forms, how it flows, and its response to probes including jets and quarkonium mesons....