  Particle colliders for high energy physics have been in the forefront of
scientific discoveries for more than half a century. The accelerator technology
of the collider has progressed immensely, while the beam energy, luminosity,
facility size and the cost have grown by several orders of magnitude. The
method of colliding beams has not fully exhausted its potential but its pace of
progress has greatly slowed down. In this paper we very briefly review the
method and the history of colliders, discuss in detail the developments over
the past two decades and the directions of the R&D toward near future colliders
which are currently being explored. Finally, we make an attempt to look beyond
the current horizon and outline the changes in the paradigm required for the
next breakthroughs.
