  We define a set of model-independent observables which generally characterize
the azimuthal dependence of two-particle correlations in nucleus-nucleus
collisions. We explain how they can be analyzed, and show to what extent such
analyses are model dependent. We discuss specific applications to the
anisotropic flow of decaying particles, azimuthally sensitive HBT, and
correlations between particles at large transverse momentum. A quantitative
prediction is made for jet quenching with respect to the reaction plane.
