Physico-chemical and chemico-petrographic investigations and geo-technical studies of stones used in ancient monuments and of the impact thereon of the ambient environment have given some insight into the weathering mechanisms and degradation phenomena of early Indian cave architecture and architectural monuments. Both physical disintegration and chemical weathering under hot-humid, hot-dry, and cold-arid conditions have resulted in deterioration of monuments in basalt, tuff, granite, gneiss, schist besides sandstone, limestone, and marble. Hydrolysis, carbonation, oxidation. Hydration, and solution are the main chemical reactions responsible for degradation resulting from kaolinization, limonitization, gypsumization and lateritization of the building stones. Insolation, wind erosion, and sand-blasting action of dust-laden winds, solution, and crystallisation of soluble salts under changing relative humidity, and the degrading action of cryptogamic growth have been identified as active and potent factors of mechanical weathering. Microbial decay of stones has not been identified at any monument, and the observed rock decay cannot be attributed to bacterial activity.