Evandro AGAZZI: Introduction. Part One: PHILOSOPHICAL CONSIDERATIONS. Paul HORWICH: Realism and Truth. Evandro AGAZZI: On the Criteria for Establishing the Ontological Status of Different Entities. Aristides BALTAS Con-straints and Resistance: Stating a Case for Negative Realism. Michel PATY: Predicate of Existence and Predictability for a Theoretical Object in Physics. Part Two: OBSERVABILITY AND HIDDEN ENTITIES. Francois BONSACK: Atoms: Lessons of a History. Alberto CORDERO: Arguing for Realities. Bernard d'ESPAGNAT: On the Difficulties that Attributing Existence to Hidden Quantities May Rise. Massimo PAURI: The Quantum, Space-Time and Observation. Part Three: APPLICATIONS TO QUANTUM PHYSICS. David ALBERT: On the Phenomenology of Quantum-Mechanical Superpositions. Gian Carlo GHIRARDI: Realism and Quantum Mechanics. Michel CROZON: Experimental Evidence of Quark Structure Inside Hadrons.