  The observation in small size collision systems, $pp$ and $p$A, of strong
correlations with long range in rapidity and a characteristic structure in
azimuth, the ridge phenomenon, is one of the most interesting results obtained
at the Large Hadron Collider. Earlier observations of these correlations in
heavy ion collisions at the Relativistic Heavy Ion Collider are standardly
attributed to collective flow due to strong final state interactions, described
in the framework of viscous relativistic hydrodynamics. Even though data for
small size systems are well described in this framework, the applicability of
hydrodynamics is less well grounded and initial state based mechanisms have
been suggested to explain the ridge. In this review, we discuss particle
correlations from the initial state point of view, with focus on the most
recent theoretical developments.
