  When a photon is sent onto an atomic ensemble, it interacts collectively with
the $N$ atoms of the sample and not simply with one of them. This results in
measurable modifications in the scattering rate, the emission diagram or the
temporal dynamics. Among these collective effects, here, we study in detail the
temporal dynamics of the decay of the scattered light after switching off the
driving laser. Right after the switch-off, the decay can be faster than for a
single atom, whereas at later time, the decay becomes slower. We refer to this
behavior as superradiance and subradiance, respectively. We present in this
Chapter our investigations of super- and subradiance in disordered cold atomic
ensembles, mainly with low-density samples (dilute regime) and a weak
excitation (linear-optics regime), but we also studied the first corrections
beyond these limiting cases. We emphasize the different possible
interpretations of these phenomena, with in particular the presentation of an
optical description of these effects, which sheds a new light on cooperative
scattering in disordered samples and provides a more intuitive understanding of
the physical mechanisms at play.
