In this chapter, the potential of micro™uidic lab-on-a-chip devices for applications involving magnetic nanoparticles will be reviewed. In Section 10.1, the concept of micro™uidics is introduced together with some underlying physical phenomena. Section 10.2 is dedicated to on-chip synthesis of magnetic nanoparticles as well as on-chip synthesis of magnetite-containing microspheres. Labon-a-chip devices may provide an alternative or complementary approach to the synthesis methods outlined in Part II of this book. Section 10.3 of this chapter concerns on-chip applications with magnetic nanoparticles. These include physical applications where magnetic nanoparticles act as pumps, valves, mixers or switches as well as bioanalytical applications such as immunoassays or sorting of magnetically labelled cells.