  We review a growing theoretical motivation and evidence that the number of
dimensions actually reduces at high energies. This reduction can happen near
the Planck scale, or much before, the dimensions that are reduced can be
effective, spectral, topological or the usual dimensions, but many things
points toward the fact that the high energy theories appear to propagate in a
lower dimensional space, rather than a higher dimensional one. We will
concentrate on a particular scenario of "vanishing" or "evolving dimensions"
where the dimensions open up as we increase the length scale that we are
probing, but will also mention related models that point to the same direction,
i.e. the causal dynamical triangulation, asymptotic safety, as well as evidence
coming from a non-commutative quantum theories, the Wheeler-DeWitt equation and
phenomenon of "asymptotic silence". It is intriguing that experimental evidence
for the high energy dimensional reduction may already exists - a statistically
significant planar alignment of events with energies higher than TeV has been
observed in high altitude cosmic ray experiments. A convincing evidence for
dimensional reduction may be found in future in collider experiments and
gravity waves observatories.
