  The main motivation to investigate hard probes in heavy ion collisions is to
do tomography, i.e. to infer medium properties from the in-medium modification
of hard processes. Yet while the suppression of high P_T hadrons has been
measured for some time, solid tomographic information is slow to emerge. This
can be traced back to theoretical uncertainties and ambiguities in modelling
both medium evolution and parton-medium interaction. Ways to overcome these
difficulties are to constrain models better and to focus on more differential
observables. Correlations of high P_T hadrons offer non-trivial information
beyond what can be deduced from single hadron suppression. They reflect not
only the hard reaction being modified by the medium, but also the back reaction
of the medium to the hard probe. Models for hard back-to-back correlations are
now very well constrained by a wealth of data and allow insights into the
nature of the parton-medium interaction as well as first true tomographic
results. Models of full in-medium jet evolution are being actively developed,
but have yet to make substantial contact with data. Progress is slower in the
understanding of low P_T correlations, the ridge and the cone, although a
qualitative understanding of the nature of the physics behind these
correlations starts to emerge.
