Immunotherapy is now one of the most promising treatment modes for cancer. Such therapy boosts the immune system to fight cancer. This chapter shows that the idea behind cancer immunotherapy has a long history and began to be developed on the back of the rise of vaccines against infectious diseases. Achieving success in the clinic, however, took a long time. It grew out of new understanding about immunology and cancer that emerged on the back of the development of new inbred strains of laboratory animals from the 1950s. As this chapter shows, the progress in immunotherapy was never inevitable or straightforward.