By pushing the energy frontier, the LHC opens a large phase-space where our beloved perturbative QCD can be reliably applied. This is true for "vacuum" pp collisions allowing, for example, for the rapid development of precision calculations and jet substructure techniques. This is also true for "medium" PbPb collisions where, for the first time one can study processes with a hard scale much larger than all the typical energy scales of the quark-gluon plasma. The main goal of this talk is to discuss two key elements of jet physics in such a perturbative phase-space: parton showers, central to our description of the high-energy collisions, and jet substructure, central to our study of jets in the final state. In both cases, I will make an attempt at discussing connections between the "vacuum" and "medium" cases as well as challenges specific to either situation.