Colossal magnetoresistance (the CMR effect) has recently been observed  in doped manganese-oxide ceramics (manganites), sparking a great amount of effort aimed at understanding the electronic and magnetic properties of these materials. At low temperatures, properly doped manganites exhibit ferromagnetic metallic or nearly metallic behavior, while at high temperatures they exhibit a paramagnetic insulating behavior. This generic behavior, as well as the magnetoresistive effect which occurs near the transition, has been understood to first order within the framework of double exchange theory, as developed in the 1950s and 60s by Zener, DeGennes, and Anderson and Hasegawa [2,3]. Recently however, there has been an increasing realization that although double-exchange is clearly important for understanding the behavior of the manganites, it is not enough and other physics must be introduced. While there has been a great amount of progress both experimentally and theoretically, it is not yet clear what the most important mechanisms are for explaining the physics of the manganites.