Massive galaxy clusters are well known to strongly affect the properties of their member galaxies, transforming star-forming spirals that have been recently arrived from the surrounding field into "red and dead" early-type galaxies. Combining wide-field Spitzer 24um and GALEX NUV imaging for 30 massive clusters at z~0.2 with extensive spectroscopy, I present a series of results which demonstrate that most (if not all) massive star-forming galaxies accreted into clusters must have their star formation slowly quenched on 1-2 Gyr time-scales. To understand how the observed trends relate to the continual accretion of star-forming spirals onto massive clusters and subsequent quenching of star-formation, I follow the infall and orbits of galaxies in the vicinity of massive clusters extracted from the Millennium cosmological simulation, obtaining a series of predicted model trends that have general applicability for understanding galaxy evolution in cluster environments. I also present results demonstrating the need for pre-processing in galaxy groups, consistent with the an over-abundance of X-ray groups in the outskirts of our cluster sample.