Following high energy physics requirements, a large R&D effort is presently developed world-wide by the accelerator physics community to define the possible colliders at the high-energy frontier for the post LHC era. Both circular and linear collider schemes are being vigorously researched for use as future multi-TeV accelerators. Conceptual designs for muon colliders and very large hadron colliders
are being investigated in detail. The development of linear colliders is well advanced, and a number of concepts have been firmly established with the different technologies being tested in large-scale test facilities.
