This volume contains the invited and contributed papers presented at the Fourth International Conference on Perspectives in Hadronic Physics and sent to the Editors within the deadline. The Conference was held at the Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy, from May 12th to 16th, 2003, and was attended by about 100 scientists from 20 countries. The series ofConferences on Perspectives on Hadronic Physics takes place every two years since 1997 and follows the seven Workshops on Perspectives in Nuclear Physics at Intermediate Energies, organized every two years at ICTP since 1983. The aim of these Conferences is to discuss the status-of-the-art concerning the experimental and theoretical investigations of hadronic systems, from nucleons to nuclei and dense nuclear matter, in terms of the relevant underlying degrees of freedom. For such a reason the Fourth Conference has been focused on those experimental and theoretical topics which have been in the last few years the object of intensive investigations, viz. the various approaches employed to describe the structure of hadrons in terms of QCD and QCD inspired models, the recent developments in the treatment of the properties and propagations of hadronic states in the medium, the relevant progress done in the solution of the few- and many- hadron problems, the recent results in the experimental investigation of dense hadronic matter and, last but not least, the physics programs of existing Laboratories and the suggested projects for new Facilities.

This book gives an introduction to main ideas used in the physics of ultra-relativistic heavy-ion collisions. The links between basic theoretical concepts (discussed gradually from the elementary to more advanced level) and the results of experiments are outlined, so that experimentalists may learn more about the foundations of the models used by them to fit and interpret the data, while theoreticians may learn more about how different theoretical ideas are used in practical applications. The main task of the book is to collect the available information and establish a uniform picture of ultra-relativistic heavy-ion collisions. The properties of hot and dense matter implied by this picture are discussed comprehensively. In particular, the issues concerning the formation of the quark-gluon plasma in present and future heavy-ion experiments are addressed.