We review our work on the characterization and modeling of the extrinsic and intrinsic contributions to the magnetoresitance of La-Ca manganites near the optimal doping. Firstly, we propose a macroscopic model for the electronic transport that parametrically quantifies the intrinsic and extrinsic contributions to the magnetoresistance, when grain boundaries are present. Then, we complete this model by considering the effects of Coulomb blockade in the electronic transport at low temperatures in polycrystalline samples. Secondly, we explore at a microscopic scale the existence of intrinsic inhomogeneities and the possibility of controlling the electron-lattice coupling by introducing controlled chemical defects. Finally, we describe the effects that these artificially built-in barriers have on the magnetoresistive properties of the system at very low magnetic fields. (C) 2003 Elsevier Science (USA). All rights reserved.