  The existence of the mysterious pseudo-gap state in the phase diagram of
copper oxide superconductors and its interplay with unconventional {\it d-wave}
superconductivity has been a long standing issue for more than a decade. There
is now a growing number of experimental indications that the pseudo-gap phase
actually corresponds to a symmetry breaking state. In his theory for cuprates,
C. M. Varma proposes that the pseudo-gap is a new state of matter associated
with the spontaneous appearance of circulating current loops within $\rm CuO_2$
unit cell. This intra-unit-cell order breaks time reversal symmetry, but
preserves lattice translation invariance. Polarized elastic neutron scattering
measurements provide evidence for an intra-unit-cell magnetic order inside the
pseudo-gap state. This order could be produced by the orbital-like magnetic
moments induced by the circulating current loops. The magnetic order displays
the same characteristic features in $\rm HgBa_2CuO_{4+\delta}$, $\rm
YBa_2Cu_3O_{6+x}$ and $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$ demonstrating that this
genuine phase is ubiquitous of the pseudo-gap of high temperature copper oxide
materials. We review the main properties characterizing this intra-unit-cell
magnetic order and discuss its interplay or competition with other spin and
charge instabilities.
