This second volume of Contemporary Topics in Immunobiology considers many aspects of thymus dependency in order to exemplify the role of the thy mus in different species and different immunological responses. It is not in tended to be a compendium of the responses which have been shown to be thymus dependent but rather to illustrate for the reader the criteria he should apply in thinking about the significance of the thymus in immune responses. We are grateful to the editors and publishers of the Annals of the New York Academy of Science, the Australian Journal of Experimental Biology and Medical Science, Clinical and Experimental Immunology, Immunology, the Journal of Experimental Medicine, the Journal of Immunology, Laboratory Investigation, Nature, and the Proceedings of the Royal Society of Medicine and to Springer-Verlag, Berlin, for permission to reproduce illustrations. Specific references are given in the text. We would also like to thank the contributors for their time and energy and willingness to submit to the editorial red pencils. The exercise of these censori ous instruments meant that the manuscripts had to be reorganized and retyped. Mrs. J. Pettis, Mrs. A. Inglefield, and Miss M. Butt all helped, and we are most grateful to them. A.J.S.D. R.L.e.