We review recent work on the preparation, characterization and application of "smart" microgel particles. A general feature of all systems under consideration here is their ability to react to external stimuli as e.g. the pH or the temperature in the system. Special emphasis is laid on our recent research work on the thermosensitive core-shell microgel particles, which are composed of a PS core and a cross-linked poly-(N-isopropylacrylamide) (PNIPA) shell. Work done on these core-shell systems is compared to developments on the investigations of similar systems. A novel synthesis method, namely photo-emulsion polymerization, has been described for the preparation of monodisperse, thermosensitive core-shell particles. Cryogenic transmission electron microscopy (cryo-TEM) has recently been employed to investigate the morphology and the volume transition of the core-shell type microgels. This method furnishes information about the thermosensitive particles that had not been available through other methods employed in previous investigations. Very recently, it has been shown that these core-shell microgels can be used as "nanoreactors" for the immobilization of metal nanoparticles. The metal nanocomposite particles show "smart" catalytic behaviour, inasmuch as the catalytic activity of nanoparticles can be switched on and off through the volume transition that takes place within the thermosensitive shell of the carrier system. We also discuss possible future applications of these systems. (c) 2007 Elsevier Ltd. All rights reserved.