  The novel phenomena observed in particle angular correlations are reviewed.
They include the double-peak away-side azimuthal correlations in relativistic
heavy-ion collisions and the long-range pseudorapidity near-side (ridge)
correlations in heavy-ion as well as in proton-induced collisions. The
collision system and energy dependence of these phenomena are examined,
wherever possible and most abundantly for the ridge correlations. Their
possible theoretical interpretations and what might be learned about the
properties of the collision systems from theoretical comparisons are discussed.
Prospective future measurements and theoretical undertakings are outlined that
might help further the understanding of the physics mechanisms underlying these
phenomena.
