  The physics of the pseudo-gap phase of high temperature cuprate
superconductors has been an enduring mystery in the past thirty years. The
ubiquitous presence of the pseudo-gap phase in under-doped cuprates suggests
that its understanding holds a key in unraveling the origin of high temperature
superconductivity. In this paper, we review various theoretical approaches to
this problem, with a special emphasis on the concept of emergent symmetries in
the under-doped region of those compounds. We differentiate the theories by
considering a few fundamental questions related to the rich phenomenology of
these materials. Lastly we discuss a recent idea of two kinds of entangled
preformed pairs which open a gap at the pseudo-gap onset temperature $T^{*}$
through a specific Higgs mechanism. We give a review of the experimental
consequences of this line of thoughts.
