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This topic develops analytical relationships and computations of power dissipation in magnetic fluids (ferrofluids) that are subject to alternating magnetic field in the radio frequency (rf) range. The ferrofluids (see Ferrofluids: Introduction) are ultrastable colloidal suspensions of magnetic nanoparticles (diameter< 15 nm) in a matrix fluid (Rosensweig 1987). The particles are prevented from agglomerating to each other by either a molecular surfactant coating or an electrical double layer repulsion. Thermal motions effectively prevent the particles from settling in applied force fields. Heating ferrofluids to achieve hyperthermia in medical treatments is an area of emerging importance.