Properties of Fossil Groups of Galaxies
  We review the formation and evolution of fossil groups and clusters from both
the theoretical and the observational points of view. In the optical band,
these systems are dominated by the light of the central galaxy. They were
interpreted as old systems that had enough time to merge all the M* galaxies
within the central one. During the last two decades many observational studies
were performed to prove the old and relaxed state of fossil systems. The
majority of these studies, that spans a wide range of topics including halos
global scaling relations, dynamical substructures, stellar populations, and
galaxy luminosity functions, seem to challenge this scenario. The general
picture that can be obtained by reviewing all the observational works is that
the fossil state could be transitional. Indeed, the formation of the large
magnitude gap observed in fossil systems could be related to internal processes
rather than an old formation.
