Following the highly successful first, second and third editions, Defence is the Best Attack 2023 will be the fourth EACR conference on this topic. In this conference, we will learn about the most recent advances in the field of immunotherapy from the basic science to clinical studies including the identification of novel targets; an improved understanding of the mechanisms of action of therapeutic interventions; as well as discussions about biomarkers of response to treatment and mechanisms of intrinsic and acquired resistance to immunotherapy.