The debate on realism in physics is usually focused on the reality of unobservable entities admitted in physical theories. This reality has been often denied (e.g., by Bas van Fraassen). The present book shows that observability is a very complex notion that does not really have direct implications on ontological issues related to the existence of the non-observable entities. This is shown through historical, philosophical and scientific considerations presented in the different parts of the book. Emphasis is also given to the role of experiments, measurement procedures and computer-analyzed data as interface between the theoretical and experimental cultures.