The book "Long-Term Modulation of Cosmic Rays" is written with objectives to provide a single source of information for new researchers in the field of Cosmic Rays as well as for P.G. and M.Phil students of Space Physics. The contents of this book are modernized and updated at various stages. In this book various topics related to cosmic ray, solar activity, long-term variation of cosmic rays in relation to solar activity has been included. The topics of recent research in this field has also been incorporated, which will be helpful to the research as well as P.G/M.Phil students at various stages.