Magnetic nanoparticles heat when exposed to an alternating magnetic field through hysteresis losses. These materials offer the potential to selectively treat cancer by heating cancer tissue from within at the cellular level. This can be successful provided a sufficient concentration of particles is achieved in the tumor possessing a sufficiently high specific absorption rate (SAR) to deposit heat quickly while minimizing thermal damage to surrounding tissue. High SAR magnetic nanoparticles have been developed comprised of iron oxide magnetic cores that have been used in mouse models of cancer. Colloidal stability of these particles, mean diameter of 80 nm, is maintained by the incorporating a polymer layer to form a "core-shell" construct. Structural, heating, and magnetic properties of these suspensions have been characterized using a variety of techniques including light and neutroon scattering. These data will be presented, as will the results of in vitro cell culture and in vivo mouse trials.