Results from the Compton Gamma-Ray Observatory (CGRO). Also includes results from missions such as HETE-II, Chandra, and XMM-Newton. In addition, results from ground-based VHE gamma-ray and radion observatories, and other ground-based and space missions related to high energy astrophysical sources. Papers include mission capabilities and anticipated science from upcoming missions such as new gamma-ray space telescopes like GLAST, Swift, INTEGRAL, and AGILE.

Gamma-ray astronomy has undergone an enormous progress in the last 15 years. The success of satellite experiments like NASA's Comp ton Gamma-Ray Observatory and ESA's INTEGRAL mission, as well as of ground-based instruments have open new views into the high-energy Universe. Different classes of cosmic gamma-ray sources have been now detected at different energies, in addition to young radio pulsars and gamma-ray bursts, the classical ones. The new sources include radio quiet pulsars, microquasars, supernova remnants, starburst galaxies, ra dio galaxies, flat-spectrum radio quasars, and BL Lacertae objects. A large number of unidentified sources strongly suggests that this brief enumeration is far from complete. Gamma-ray bursts are now estab lished as extragalactic sources with tremendous energy output. There is accumulating evidence supporting the idea that massive stars and star forming regions can accelerate charged particles up to relativistic ener gies making them gamma-ray sources. Gamma-ray astronomy has also proved to be a powerful tool for cosmology imposing constraints to the background photon fields that can absorb the gamma-ray flux from dis tant sources. All this has profound implications for our current ideas about how particles are accelerated and transported in both the local and distant U niverse. The evolution of our knowledge on the gamma-ray sky has been so fast that is not easy for the non-specialist scientist and the graduate student to be aware of the full potential of this field or to grasp the fundamentals of a given topic in order to attempt some original contribution.

This book provides a theoretical and observational overview of the state of the art of gamma-ray astrophysics, and their impact and connection with the physics of cosmic rays and neutrinos. With the aim of shedding new and fresh light on the problem of the nature of the gamma-ray sources, particularly those yet unidentified, this book summarizes contributions to a workshop that continues today.

After describing cosmic gamma-ray production and absorption, the instrumentation used in gamma-ray astronomy is explained. The main part of the book deals with astronomical results, including the somewhat surprising result that the gamma-ray sky is continuously changing.

Nearly one half of the point-like gamma-ray sources detected by EGRET instrument of the late Compton satellite are still defeating our attempts at identifying them. To establish the origin and nature of these enigmatic sources has become a major problem of current high-energy astrophysics. The second workshop on Multiwavelength Approach to Unidentified Gamma-ray Sources intends to shed new and fresh light on the problem of the nature of the unidentified gamma-ray sources. The proceedings contain 46 contributed papers in this subject, which cover theoretical models on gamma-ray sources as well as the best multiwavelength strategies for the identification of the promising candidates. The topics of this conference also include energetic phenomena occurring both in galactic and extragalactic scenarios, phenomena that might lead to the appearance of what we have called high-energy unidentified sources. The book will be of interest for all active researchers in the high-energy astrophysics and related research areas as well as for scientists and graduate students interested in understanding the recent progress in high-energy astrophysics.

Discussing results from the nine year operation of the Compton Gamma-Ray Observatory, this text also includes results from contemporary missions such as HETE-II, Chandra, and XMM-Newton in addition to ground-based VHE gamma-ray and radion observatories.

The Energetic Gamma-Ray Experiment Telescope (EGRET) instru ment on the Compton Gamma-Ray Observatory left as a legacy its Third Catalog of High Energy Gamma-Ray Sources, whose detections include a large number of blazars, some pulsars, the Large Magellanic Cloud and a solar flare. Most of the newly discovered objects - a majority of the catalog -are unidentified sources, with a clearly predominant Galactic population. Are all these radio-quiet pulsars, like Geminga, or is there a novel type of celestial object, awaiting identification? In spite of the limited angular resolution provided by EGRET and COMPTEL, there is still much to learn about unidentified ,-ray sources: correlation studies, multiwavelength observations and theoretical work can provide valuable clues, specially if these efforts are carried out in a coordinated manner. The aim of this workshop, held from October 9 to 11, 2000, at the Instituto N acional de Astrofisica, Optica y Electronica, at Tonantzintla, Mexico, was to gather experts on the subject, including observational as tronomers specialized in other regions of the electromagnetic spectrum, in an effort to address the question of the Nature of Galactic high-energy gamma-ray sources, both from the theoretical and observational perspec tive, and elaborate schemes for future identification studies which can make use of existing and forthcoming facilities.