This book contains the proceedings of a conference devoted to the study of the structure and function of Terminal deoxynucleotidyl Transferase (TdT) and its utilization as biochemical marker in immuno­ biology and leukemia, held in Elba, Italy on May 28-31, 1981. The en­ zyme has been known to nucleic acid biochemists for more than 20 years and has proved to be an excellent tool for making deoxypolymers, label­ ing DNA fragments, and adding homopolymer tails to restriction endonu­ clease fragments from DNA. Since the discovery of its peculiar tissue distribution, normally restricted to the thymus and bone marrow, and of its abnormal occur­ rence in human leukemic lymphoblasts, TdT has become one of the most widely used markers in the study of lymphocyte differentiation and in the classification of hematopoietic neoplasia. The subject seemed most appropriate for a meeting where molecular and cellular biologists, immunologists and hematologists could convene for the first time to discuss both basic research and the clinical aspects of the problem. Among the goals achieved by this workshop was the sharing of in­ formation about the enzymology of TdT, biochemical and immunological methodology and the correlation of TdT with other markers in the diag­ nosis of leukemia. The remarkable accordance of results, presented here by five independent hematological institutions, from analysis of TdT in several thousands of leukemic patients, marks the importance of this enzyme as a diagnostic and prognostic tool in these diseases.