I review recent selected developments in the theory and modeling of ultrarelativistic heavy-ion collisions. I first show selected recent data from the LHC. I use them as illustrations to introduce phenomena which are specific to nucleus-nucleus collisions at high energies, namely, jet quenching and collective flow. I show that our qualitative understanding of these phenomena has made significant progress over the last two years. I then describe how quantitative prediction is made: I explain why relativistic viscous hydrodynamics is now used to model the expansion of the matter formed in these collisions. I give examples of prediction for nucleus-nucleus collisions at the LHC. I show how they compare with the first data, and what they can tell us about the properties of the quark-gluon plasma.