Revision with unchanged content. Studies on the Indian summer monsoon have a long history, ascending back to the early 20th century. However, causes for year-to year variations of the Indian summer monsoon rainfall, that is of particular importance for India's society, still remain unsolved. In addition to that, the climate over the Indian Ocean shows a drastic change in the late 20th century as can be seen in the warming of the Indian Ocean sea surface temperatures. Statistically analyzing observational data from the late 19th throughout the 20th century, the relation between the Indian summer monsoon rainfall and Indian Ocean climate, and the secular change in the Indian Ocean condition are delineated in this book. Professionals on Climate Sciences, Paleoclimate and Oceano-graphy, or anyone else who is interested in the environment of the tropical Indian Ocean region will find the results of this book useful.