The Winter School "Nuclear Matter and Heavy Ion Collisions", a NATO Research Workshop held at Les Houches in February 89, has been devoted to recent developments in nuclear matter theory and to the study of central heavy ion collisions in which quasi macroscopic nuclear systems can be formed at various temperatures and densities. At in cident energies below 100 Me V per nucleon, the kinematic conditions are favourable for producing transient hot nuclei with temperatures of the order of a few MeV. At higher ener gies (100 MeV < E/A < 1-2 GeV) heavy ion collisions offer the possibility of investigating the properties of hot and dense nuclear systems. The Workshop has been motivated by important theoretical developments in transport equations whicll make it possible to relate microscopic descriptions of heavy ion collisions to nuclear matter theory and by the need to review the large body of data available on heavy ion collisions and discuss future experimental programs. This discussion was especially timely a few months before the new SIS/ESR Heavy Ion Fa.cility starts operating in Darmstadt. The School consisted mostly of series of lectures on nuclear matter, transport equations and the dynamics of heavy ion collisions. The data and their interpretation were exten sively discussed; the information carried by the various types of particles emitted during the collisions (photons, lepton pairs, pions, hons, nucleons, fragments) has been particu larlyemphasized. Specialized topics were presented as shorter contributions by participants.