The papers that make up this book were contributed by a cluster of luminaries&-John A. Wheeler, Fred Hoyle, H. Arp, and others of their magnitude discuss topics central to their various research interests. Originally given as lectures at the 1972 summer session of the International School of Astrophysics, held in Erice, Sicily, the papers have since been edited to bring them into consistent format for book publication. The thrust of the volume as a whole is that a closer symbiotic relationship should be established between high energy astrophysics and elementary particle physics&-that each has much to contribute to the other. By keeping close track of the discoveries of particle production and decay made on earth, the astrophysicist might be prompted to look for corresponding phenomena in deep space; or his research might be guided by current developments in field theory that have universal applications. Likewise, astrophysics gives to the particle physicist a new setting and high energy laboratory in which elementary particle and field theoretic ideas might be tested or further extended. New processes could be uncovered that do not reveal themselves within the severely restricted energy limits of earth-based apparatus, and phenomena that by their nature exist only on a macroscopic scale might be detected. In this regard, the book includes a discussion of the constraints on fundamental physics set by the macroscopic world as a whole. The simple application of known physical processes in elementary particle physics to astrophysics (for example, gamma-ray production in pion decay), which has been the subject of several other publications, has been deemphasized in this volume in favor of extending known particle physics into new domains. Included are discussions of quantum gravity, the mass spectrum of elementary particles at high energies, ko decay, and particle cosmology. The subjects taken up and their authors are as follows: QSO's, observations, and the problem of the redshift in astronomy (H. Arp)&-Cosmology (K. Brecher)&-Observational problems of high energy astrophysics (E. M. Burbidge)&-Theoretical problems of high energy astrophysics (G. R. Burbidge)&-Observational x-ray astronomy (R. Giacconi)&-Statistical thermodynamics of strong interactions and cosmology (R. Hagedorn)&-Connections of microphysics to cosmology and high energy astrophysics (F. Hoyle)&-Electrodynamics and cosmology (J. Narlikar)&-Weak interactions, lagging cores, and cosmology (Y. Ne'eman)&-Structure and hydrodynamics of galaxies (K. Prendergast)&-Strong interactions, gravitation, and cosmology (A. Salam)&-Extragalactic observational astronomy (W. Sargent)&- Theoretical x-ray astronomy (G. Setti)&-General relativity collapse and singularities (J. A. Wheeler).