This book is a collection of comprehensive reviews on astrophysics at the highest energies. It puts together, for the first time, discussions of astrophysics from MeV to EeV energies and beyond. Observations at these energies reveal nuclear and particle physics throughout our galaxy as well as in the most extreme environments in the entire cosmos. These reports range from the recent spectacular results from the Compton Gamma Ray Observatory, including the latest information on enigmatic gamma ray bursts, to cosmic rays at the highest energies ever observed by man.