This volume contains the proceedings of the NATO Advanced Study Institute held in Porto Conte (Alghero), Sardinia, September 15-27, 1991. The A. S. 1. was attended by 86 graduate and postgraduate students from 18 different countries, and was hosted by the newly founded International Laboratory of Molecular Genetics of Porto Conte, directed by Prof. Marcello Siniscalco. The A. S. I. was funded by NATO Scientific Affairs Division, the International Union of Immunological Societies, the European Community (Directorate General for Science, Research and Development), the Italian Research Council, and the San Raffaele Institute of Milano. In addition, a number of students who reside in the U. S. received travel funds from the U. S. National Science Foundation, and the Turkish National Fund provided financial assistance to several students from Turkey. When we decided to organize a course on T lymphocytes, our concern was to reach a balance between the teaching of both the hard core principles and the latest experimental findings of cellular immunology, and the recently expanded interfaces with the not-yet- known: hypotheses, spe CUlations, new projections to be born from the discussions.