The 2027 European Symposium on the Immune System of Bacteria (SISB2027) will bring together leading researchers to explore the rapidly expanding field of bacterial immunity. The programme will cover three major themes: the discovery and classification of novel defence systems; the molecular and structural mechanisms underlying defence and anti-defence strategies; and the ecology and evolution of immune interactions between bacteria, phages, and other mobile genetic elements. The symposium will also examine how insights from bacterial immunity are transforming our understanding of immune systems across the tree of life. SISB2027 aims to strengthen the international bacterial immunity community, support early-career researchers, and foster new collaborations through a highly interactive and networking-focused programme.