This is a collection of technical papers in the foundations and the philoso­ It takes both "foundations" phy of physics with emphasis on the former. and "philosophy" in their narrow technical senses but it construes "physics" lato sensu, as including all the sciences of nonliving systems. All eleven papers constituting this volume were written for it. The problems tackled in this book concern certain basic concepts, hypotheses, theories, and research programmes in physical science. Some of these problems are topical, others new, but they are all fundamental and the subject of research and controversy. Consequently this volume is expected to serve those students, teachers and researchers who enjoy learning, teaching, discussing or doing theoretical physics. It is addressed to the nine to niners rather than to the nine to fivers. It is expected to attract the theoretician in search for new basic ideas, the teacher eager to perfect his understanding of physical theory and transmit his own zeal and his own doubts, as well as the student anxious to get down to essentials. This book may also interest the mathematician for whom physics offers a challenge (or a good pretext). Finally, it should get the attention of the philosopher of science aware of the advantages of philosophizing on foundations research problems rather than on the popularization of some results of research. There are at least two reasons for valuing foundations research.