  X-ray observations indicate that non-gravitational processes play a key role
in the thermodynamics of the Intra Cluster Medium (ICM). The effect of
non-gravitational processes is imprinted in the ICM as an entropy minimum,
whose effects are visible in the Luminosity-Temperature relation and in the
Entropy-Temperature relation. However, the X-ray emission alone cannot
discriminate between different mechanisms and sources of heating. There are no
answers at present to the following questions: how much non-gravitational
energy per baryons is present in the ICM? When was this energy injected? Which
are the sources of heating? The embarrassment in front of these questions is
amplified by the fact that the most viable sources of heating, SNae and stellar
winds, seem to be inefficient in bringing the ICM to the observed entropy
level. We may call it the energy crisis. Here we review the main aspects of
this crisis, listing possible solutions, including other sources, like AGNs and
Radio Galaxies, or other mechanisms, like large scale shocks and selective
cooling.
