Immunotherapy refers to harnessing the immune system to treat cancer and infectious diseases. The immune system is robust enough to fight microbes and also perform what is termed as immune surveillance or checking for any alterations in cells. The idea of immunotherapy is hence simple-it activates the immune system to target pathogens or cancer cells. Given these interesting aspects, this book covers the history and basics of immunotherapy. Apart from discussing the approaches, the aspects of challenges are presented. These include honing the specificity and the cost of the therapy. Potential solutions to overcome these challenges have been presented. Thus, a reader can read this concise book that presents the fundamentals of immunotherapy without getting overwhelmed.