The human immune system is a complex network of tissues and organs dispersed throughout the body. Immunology, as one of the most rapidly evolving fields in bio¬medical research, has to date covered the essential cellular and molecular events neces¬sary for immune responses to occur. However, it has paid relatively little attention to important developmental processes underlying the formation of the tissues themselves that carry out immune responses in humans and other mammalians. In contrast to the thymus and bone marrow that are the sole tissues for generating mature leukocytes for antigen recognition and han¬dling in humans and most mammalian species, the peripheral lymphoid tissues where adaptive immune responses are focused display broad tissue distribution and possess diverse archi¬tectural characteristics. These organs develop prior to the individual's exposure to external antigens, and despite their similar functions, their varied appearances indicate a substantial complexity of tissue ontogeny. This volume presents a comprehensive overview of the developmental features of the major peripheral lymphoid organs, thus examining the connection between immunological functionality and structural characteristics utilizing a developmental approach, for an audience ranging from undergraduate students to senior researchers in immunology, histology and clinical medicine.