ABSTRACTVery large increase in electrical conductivity by several orders of magnitude is obtained when an external magnetic field is applied to the colossal magnetoresistance (CMR) materials such as La-Ca-Mn-O. The magnetoresistance is strongly temperature-dependent, and exhibits a sharp peak below room temperature, which can be shifted by adjusting the composition or processing parameters. The control of lattice geometry or strain, e.g., by chemical substitution, epitaxial growth or post-deposition anneal of thin films appears to be crucial in obtaining the CMR properties. The orders of magnitude change in electrical resistivity could be useful for various magnetic and electric device applications.