What hard probes tell us about the quark-gluon plasma: Theory

In the study of the quark-gluon plasma in high-energy heavy-ion collisions, hard and electromagnetic (EM) processes play an essential role as probes of the properties of the dense medium. They can be used to study a wide range of properties of the dense medium in high-energy heavy-ion collisions, from space-time profiles of the dense matter, bulk transport coefficients to EM responses and the jet transport parameter. I review in this talk these medium properties, how they can be studied through hard and EM probes and the status of recent theoretical and phenomenological investigations.