  Muon colliders have a great potential for high-energy physics. They can offer
collisions of point-like particles at very high energies, since muons can be
accelerated in a ring without limitation from synchrotron radiation. However,
the need for high luminosity faces technical challenges which arise from the
short muon lifetime at rest and the difficulty of producing large numbers of
muons in bunches with small emittance. Addressing these challenges requires the
development of innovative concepts and demanding technologies. The document
summarizes the work done, the progress achieved and new recent ideas on muon
colliders. A set of further studies and actions is also identified to advance
in the field. Finally, a set of recommendations is listed in order to make the
muon technology mature enough to be favourably considered as a candidate for
high-energy facilities in the future.
