This chapter discusses the cellular composition of lymph beginning with a historical perspective into the evolution of our understanding of lymph and the diversity of cells that traffic within it. The biological roles and importance of immune cell transit through the lymphatic vasculature is reviewed in the context of immunity, but with emphasis on new emerging information delineating its importance for the regulation of the immune system. Furthermore, the chapter highlights new insights into how immune cell traffic through lymph influences the function of the entire lymph node itself. The disequilibrium of lymphocyte subtypes in afferent and efferent lymph is discussed in light of the importance these trafficking patterns have on immunity, immunological memory, and the maintenance of tolerance to self-antigen. Trafficking of dendritic cells through the lymphatic vasculature, the importance of delivering these cells to the correct locale, and the mechanisms governing this process are all discussed in detail.