One of the most active areas of research in condensed matter physics studies the origin of large magnetoresistance effects in particular materials. Among them, the Mn oxides widely known as manganites are much analyzed due to the existence of a phenomenon called ``colossal magnetoresistance'' (CMR), where the resistance of the sample   changes dramatically -- by many orders of magnitude --   once relatively small fields are turned on. The magnetoresistance ratio [R(0)-R(H)]/R(H) can be as large as 100,000%. An example, from the group of Y. Tokura with the University of Tokyo, is shown for PrCaMnO. These materials have potential applications as read sensors in computers.   Considerable theoretical and experimental work in this area of research has converged to a view of the state that has the CMR effect as made out of nanoscale islands of the two leading competing states: ferromagnetic metallic (FM) and   antiferromagnetic (AF) insulating. A sketch is shown in the figure, where the arrows denote the randomly oriented FM clusters, which are separated by walls made out of the AF state.   Several groups at ORNL and UT are actively working in projects that will help in the understanding of these compounds, using a variety of techniques. A review of manganites can be found here. Mn oxides are routinely studied using neutron scattering techniques, both elastic and inelastic, while Monte Carlo simulations address the origin of the effect employing the excellent computational facilities at the Condensed Matter Science Division (cluster of PCs) and at the Computational Sciences Division (a variety of supercomputers).