1. Introduction.- 2. Phylogenetic and Ontogenetic Considerations.- 2.1. Phylogeny of Acquired Immunity.- 2.2. Ontogenetic Development of the Lymphoreticular Tissue and of Acquired Immunity.- 3. Growth Characteristics and Cellular Kinetics of the Thymus.- 3.1. Growth Characteristics of the Thymus.- 3.2. Proliferation of Thymic Lymphoid Cells.- 3.3. Intrathymic Fate of Newly Produced Lymphoid Cells.- 3.3.1. Intrathymic Cell Death.- 3.3.2. Migration of Lymphoid Cells from the Thymus.- 4. The Effects of Thymectomy on Antibody Formation.- 4.1. Neonatally Thymectomized Animals.- 4.1.1. Conventionally Raised Animals.- 4.1.1.1. Mice.- 4.1.1.2. Species other than Mice.- 4.1.2. Specific Pathogen-free and Germfree Animals.- 4.1.3. Discussion of Antibody Formation in Neonatally Thymectomized Animals.- 4.2. Animals Thymectomized in Adult Life.- 4.3. Thymectomy and Acquired Immunological Tolerance.- 5. Additional Examples of Acquired Immunity in Thymectomized Animals.- 5.1. Hypersensitivity Reactions.- 5.2. Transplantation Immunity.- 5.3. Graft-versus-host Reactions.- 6. Thymectomy and Bursectomy in Birds.- 7. Post-thymectomy Wasting.- 8. Morphological and Functional Lymphoid Reconstitution of Thymectomized Animals.- 8.1. Thymus Implants.- 8.2. Injection of Lymphoid Cells.- 8.3. Diffusion Chamber Implants Containing Lymphoid Cells.- 8.4. Summary.- 9. The Role of the Thymus in Immunologic Deficiency Disorders in Man.- 10. Conclusions.- Literature.