  In this proceedings contribution I review recent progress in our
understanding of the bulk dynamics of relativistic systems that possess
potentially large local rest frame momentum-space anisotropies. In order to
deal with these momentum-space anisotropies, a reorganization of relativistic
viscous hydrodynamics can been made around an anisotropic background, and the
resulting dynamical framework has been dubbed "anisotropic hydrodynamics." I
also discuss expectations for the degree of momentum-space anisotropy of the
quark gluon plasma generated in relativistic heavy ion collisions at RHIC and
LHC from second-order viscous hydrodynamics, strong-coupling approaches, and
weak-coupling approaches.
