This volume contains the lectures presented by invited speakers at the IV La Rlibida International Summer School on Nuclear Physics. This was the IV edition of a summer school organized by our group every three years on topics related to nuclear physics. This Summer School was aimed mainly at young nuclear physicists, both theoreticians and experimentalists, engaged in research work at predoctoral or recent postdoctoral level. The topics treated in the three previous editions of the School were: "Heavy Ion Collisions", "Theory of Nuclear Structure and Reactions" and "Nuclear As trophysics". This year's School was entitled "Nuclear Physics at the Borderlines". Special emphasis was placed on those topics along which nuclear physics is ex pected to develop in the next few years. The aim of the School was to provide the attendants with an opportunity to get into close contact with experienced researchers and listen to their account of the present state-of-the-art in nuclear physics and the main future lines of development.

This proceedings contains the talks delivered at the plenary and parallel sessions. Topics covered include e⁺e⁻ Physics at Z0, String Theory and Theory of Extended Objects, High Energy pp Physics, Non-Accelerator Particle Physics, Conformal Field Theory, e⁺e⁻ Physics below Z⁰, Structure Functions and Deep Inelastic Scattering, Neutrino Physics, Recent Developments in 2-Dimensional Gravity, Lattice Gauge Theory and Computer Simulations, CP Violation , Accelerator Physics, Cosmology and Particle Physics, Interface Between Particle and Condensed Matter Physics, Detector R&D, and Astroparticle Physics.

During July 1990. a group of 83 physicists from 43 laboratories in 21 countries met in Erice for the 28th Course of the International School of Subnuclear Physics. The countries represented were: Algeria. Canada. Chile. China. Czechoslovakia. Denmark. France. the Federal Republic of Germany. Greece. Holland. India. Italy. Pakistan. Peru. Poland. Sweden. Switzerland. Turkey. the Union of Soviet Socialist Republics. the United Kingdom. and the United States of America. The School was sponsored by the European Physical Society (EPS). the Italian Ministry of Education (MPI). the Italian Ministry of University and Scientific Research. the Sicilian Regional Government (ERS). and the Weizmann Institute of Science. The new topic discussed in some detail at the School was QCD phenomenology at 200 TeV. This energy frontier represents the goal for subnuclear physics after LHC and sse. The main lecturers were T.D. Lee. L. Cifarelli. Y. Dokshitzer and A. Ringwald. There are two possibilities. one optimistic. the other pessimistic. LHC and SSC can produce remarkable discoveries. In this case. the need to go to higher energies will be obvious. In the pessimistic case. LHC and SSC will produce the strongest evidence for the validity of the Standard Model. The need for higher energies will be impelled. In neither case can we wait. If the 200 TeV frontier is to be reached within our lifetime we need to start now: thinking and preparing the tools. Theory and R&D for future detectors are the basic points of the 200 TeV frontier.

This 2002 monograph, now reissued as OA, explores the primordial state of hadronic matter called quark-gluon plasma.