Manganese oxides exhibiting colossal magnetoresistance (CMR) fall broadly into two structural categories : the substituted rare-earth manganite perovskites (Ln(1-x)D(x)MnO(3)) and the thallium manganite pyrochlore (Tl2Mn2O7). The driving forces for the CMR transition in perovskites are thought to be the double-exchange interaction resulting from Mn3+/Mn4+ mixed valency as well as polaronic effects resulting from strong electron-phonon coupling due to the presence of Jahn-Teller ion, Mn3+. On the other hand, structural determination from neutron and single-crystal X-ray diffraction data as well as X-ray absorption spectroscopic analysis of Tl2Mn2O7 pyrochlore show no deviations from ideal stoichiometry, Mn3+/Mn4+ mixed valency or Jahn-Teller distortions of the MnO6 octahedron indicating that the driving force for the CMR here is fundamentally different from perovskites. We review the similarities and the differences in structural and electronic properties exhibited by these two families, and discuss the implications of these observations to our understanding of the mechanism of CMR in these materials.