The lesion of rheumatoid arthritis (RA) is a tissue infiltrate of inflammatory cells accumulating in the synovial membrane. Tissue-infiltrating cells are lymphocytes and macrophages; neutrophils are rarely seen in rheumatoid synovitis. Professional antigen-presenting cells are also represented in the infiltrates, providing the opportunity for adaptive immune responses to occur at this unusual tissue site. While the identification and characterization of antigens presented and recognized in the synovial layer is of obvious interest, it is equally important to understand the factors in the synovial microenvironment that facilitate immune recognition. Optimal immune responses require not only the interactive effort of several different and highly sophisticated cell populations, but also depend upon the spatial relationship of cells involved in the generation of immune responses. The immune system has accommodated for this need by the formation of primary, secondary, and tertiary lymphoid structures. A unique feature of the immune system is that antigens are transported to lymphoid tissues where appropriate cell densities and the proper architecture are available for priming of naive T cells and B cells. In contrast to many other chronic inflammatory diseases, the lymphoid infiltrates in rheumatoid synovium establish a complex arrangement and actually form microstructures of tertiary lymphoid tissues. The formation of extranodal lymphoid tissues in synovial membrane cannot remain without consequences and raises several important questions that will be addressed in this review. What is unique about the layer of synovial lining that allows for the recruitment of lymphocytes and macrophages and also for the generation