Substituted simple and double perovskite-structure based manganites exhibit two kinds of magnetoresistance (MR): Intrinsic or intragrain MR in the vicinity of Curie temperature (T-c) and extrinsic or intergrain below T-c. The intrinsic-MR is observed at large magnetic fields (in Tesla range) whereas an extrinsic MR which originates from the interfaces of naturally occuring or artificially created grain boundaries is exhibited even at low magnetic fields. Low field magnetoresistance (LFMR) is exhibited below T-c and decreases rapidly with increasing temperature. Recent experimental developments of LFMR in single and double perovskite-structure manganites are summarized. Models of the mechanism of LFMR based on the spin dependent transport across the grain boundaries are reviewed. A better understanding of the nature of grain boundaries and transport mechanism across them is necessary to enhance LFMR at higher temperatures for the applications. Investigations of the substitution of mono- di- and trivalent metals such as Ag+, Zn2+ and Au3+ in the voids and at the grain boundaries of a polycrystalline manganite La0.5Pb0.5MnOx and it's influence on the LFMR is described.