  I review recent selected developments in the theory and modeling of
ultrarelativistic heavy-ion collisions. I explain why relativistic viscous
hydrodynamics is now used to model the expansion of the matter formed in these
collisions. I give examples of first quantitative predictions, and I discuss
remaining open questions associated with the description of the freeze-out
process. I argue that while the expansion process is now well understood, our
knowledge of initial conditions is still poor. Recent analyses of two-particle
correlations have revealed fine structures known as ridge and shoulder, which
extend over a long range in rapidity. These correlations are thought to
originate from initial state fluctuations, whose modeling is still crude. I
discuss triangular flow, a simple mechanism recently put forward, through which
fluctuations generate the observed correlation pattern.
