The immune system is the complex set of biological defenses that fight infections and other diseases such as cancer. Many aspects of the immune system are not yet completely understood and mathematical models shed light on how the different subsystems interact and develop specific immune responses. However, the immune system does not always eliminate tumor cells, as these develop mechanisms to evade it. Immunotherapies stimulate the immune system to increase the body’s antitumor response. Mathematical models help in understanding the details of the actions of immunotherapies, in developing image-based biomarkers of response, and in designing more effective therapeutic approaches.