Contributors were asked to emphasize their own experience with nanoparticles or microspheres. They were also encouraged to provide aspects normally not stipulated in specialized reviews. The objective was to produce a book which although written by specialists, presented a subject which could be easily understood by people in different fields of life sciences. Another goal was to set up a condensed work covering many aspects of nanoparticles and microspheres. That is the reason why, although mainly dealing with synthetic drug carriers, the first chapter is related to preparation and physicochemical properties of nanoparticles constructed from natural polymers. This is followed by two chapters involving the application of polymeric drug carriers in cancer therapy. The second chapter extensively describes in vitro and in vivo behaviour of purely synthetic nanoparticles. As ub the case of liposomes, a problem can be that some particles are preferentially taken up by the reticuloendothelial system. Magnetic drug carriers provide a partial answer to this problem since their tropism can be managed by an external magnet. The third chapter is entirely devoted to this aspect. A brief but original application of nanoparticles is given in the fourth chapter by reviewing their use in ocular therapy. Finally, two general applications of polymeric microspheres are discussed in chapters 5 and 6.