  Over the past two decades, the high energy physics community has been
actively discussing and developing a number of post-LHC collider projects;
however, none of them have been approved due to high costs and the uncertainty
in post-LHC physics scenarios. There have been great expectations of rich new
physics in the 0.1-1 TeV mass region: the Higgs boson (one or several),
supersymmetry, or perhaps new particles from the dark-matter family. It has
been the general consensus that the best machine for the detailed study of new
physics to be discovered at the LHC would be an energy-frontier linear e+e-
collider. Physicists held their breath waiting for the results from the LHC. In
summer 2012, two LHC detectors, ATLAS and CMS, announced the discovery of a
Higgs boson with the mass of 126 GeV and (still) nothing else. The absence of
new physics in the region below 1 TeV has changed the post-LHC collider R&D
priorities and triggered a zoo of project proposals for the precision study of
the 126 GeV Higgs boson, possibly with further upgrades to higher energies.
This paper gives an overview of these projects; it is based largely on the
reports presented at the first workshop on Higgs factories held at FNAL a few
days prior to the present workshop in Protvino.
