from normal spirals by their higher bulge luminosities rather than fainter disks [@CZ04], disfavouring simple RPS or starvation mechanisms [but see @KSS09]. Instead, mechanisms such as merging or harassment are capable of channelling material to a central bulge, sufficient to produce the higher central mass densities seen in cluster spirals and ultimately the stellar phase densities found in S0s [@MMT07]. This channeling of material will also likely fuel rapid growth of the central supermassive black hole and trigger a period of nuclear activity. Deep observations are needed to reveal tidal streams or ‘fans’, characteristic of recent merging activity [@vD05].

In-situ transformation via merging in clusters is strongly suppressed due to the high encounter velocities. Low-velocity encounters and mergers should be much more frequent in galaxy groups, leading to the suggestion that the bulge growth required to form S0s occurs primarily via pre-processing in galaxy groups, which are subsequently accreted into clusters [@BEM04]. Moreover, the S0 fractions of $z{\la}0.5$ groups match those seen in clusters at the same redshifts, and are much higher than found in the field [@WOM09]. Strong nuclear activity which is linked to bulge growth is also found to be effectively suppressed in cluster galaxies, with the distribution 