Various aspects of 'spillover' and 'reverse spillover' in catalysis are considered. The ideas of Kisliuk regarding the kinetic role of weakly held adsorbed states as precursors to stronger chemisorption are shown to be important for reverse spillover, as highlighted by Boudart's concept of the 'collection zone'. STM has proved to be an important tool for the understanding of the formation and thermal evolution of nanoparticles on surfaces. Here we present direct, in situ observations of spillover for the reaction of oxygen with Pd nanoparticles on TiO2(110). In turn, this is connected to a form of SMSI, and it is proposed that this spillover may also be the mechanism of the oxygen storage phenomenon for CeO2-doped automobile catalysts.