A brief theory overview of open heavy quarks is presented in this work. Different theories and models are summarized for heavy quark energy loss and hadronization inside the QGP. Two newly developed multi-scale evolution approaches are then introduced for heavy quark energy loss, which combine heavy quark interaction with the medium at different virtuality scales, and different scales of the momentum exchange. In the end, recent theoretical progress on probing the medium properties using heavy quarks has been presented, including probing the QGP evolution history and probing the initial state effects in high-energy nuclear collisions.