Thymic B cells are a unique population of B lymphocytes that reside at the cortico-medullary junction of the thymus, an organ that is specialized for the development and selection of T cells. These B cells are distinct from peripheral B cells both in terms of their origin and phenotype. Multiple lines of evidence suggest that they develop within the thymus from B lineage-committed progenitors and are not recirculating peripheral B cells. Furthermore, thymic B cells have a highly activated phenotype. Because of their location in the thymic medulla, they have been thought to play a role in T cell negative selection. Thymic B cells are capable of inducing negative selection in a number of model antigen systems, including viral super antigen, peptides from immunoglobulin, and cognate self antigen presented by B cell receptor-mediated uptake. These findings establish thymic B cells as a novel and important population to study; however, much work remains to be done to understand how all of these unique aspects of thymic B cell biology inform their function.