  We report on the observation of collective atomic recoil lasing and
superradiant Rayleigh scattering with ultracold and Bose-Einstein condensed
atoms in an optical ring cavity. Both phenomena are based on instabilities
evoked by the collective interaction of light with cold atomic gases. This
publication clarifies the link between the two effects. The observation of
superradiant behavior with thermal clouds as hot as several tens of
$\mu\textrm{K}$ proves that the phenomena are driven by the cooperative
dynamics of the atoms, which is strongly enhanced by the presence of the ring
cavity.
