Publisher Summary This chapter discusses the fluid dynamics and science of magnetic liquids. Fluid media composed of solid magnetic particles of subdomain size colloidally dispersed in a liquid carrier are the basis for the highly stable, strongly magnetizable liquids known as magnetic fluids or ferrofluids. The number density of particles in suspension is in the order of lO23/m3. It is the existence of these synthetic materials that makes the study of magnetic liquid fluid dynamics (ferrohydrodynamics) possible. There are two broad ways to make a magnetic fluid: size reduction of coarse material and chemical precipitation of small particles. Size reduction has been done by spark evaporation–condensation, electrolysis, and grinding. Chemical routes include decomposition of metal carbonyls and precipitation from salt solutions. The fluid dynamics of magnetic fluids differ from that of ordinary fluids in that stresses of magnetic origin appear and, unlike in magnetohydrodynamics, there need not be electrical currents. Magnetic fluid pushes the nonmagnetic fluid in the presence of tangential applied field. The fluid motion is normal to the interfacial boundary between the two fluids.