Tertiary lymphoid structures (TLS) are ectopic lymphoid formations found in inflamed, infected, or tumoral tissues. They exhibit all the characteristics of structures in the lymph nodes (LN) associated with the generation of an adaptive immune response, including a T cell zone with mature dendritic cells (DC), a germinal center with follicular dendritic cells (FDC) and proliferating B cells, and high endothelial venules (HEV). In this review, we discuss evidence for the roles of TLS in chronic infection, autoimmunity, and cancer, and address the question of whether TLS present beneficial or deleterious effects in these contexts. We examine the relationship between TLS in tumors and patient prognosis, and discuss the potential role of TLS in building and/or maintaining local immune responses and how this understanding may guide therapeutic interventions.