  Hard processes leading to high transverse momentum hadron production are
calculable in perturbative Quantum Chromodynamics (pQCD) for proton-proton
collisions. In heavy-ion collisions, such processes occur as well, and due to a
separation of scales are calculable on the level of the production of high p_T
partons. These subsequently interact with the medium which originates from
semi-hard and soft processes in the same collision before they hadronize. The
presence of the medium thus modifies the momentum spectrum of outgoing partons
and thus hard hadrons. The idea of jet tomography is to study this modification
in order to learn about the medium density distribution. While this is a
compelling idea, its practical application encounters some difficulties. In
this paper, the capability of various observables to provide tomographic
information about the medium is discussed and compared.
