High-energy astrophysics studies the Universe at the extreme. Black holes, neutron stars, exploding supernovae, and relativistically moving jets continually challenge our understanding of the behavior of matter at extreme densities and temperatures, high velocities, enormous magnetic fields, and strong gravity. Understanding these extreme environments is key to interpreting the bizarre energetic phenomena that occur in Active Galactic Nuclei, pulsars, supernovae, and gamma-ray bursts. The field of high-energy astrophysics is currently experiencing an explosion in data quality and in the level of sophistication of the modeling. For the next several years, we expect to experience a unique confluence of simultaneous observations from the ground and space-based telescopes that span the whole electromagnetic spectrum: JVLA, ALMA (radio and sub-millimeter), Hubble/JWST (optical/infrared), Chandra, SWIFT, NuStar (X-rays), INTEGRAL and Fermi (gamma-rays), and HESS/MAGIC (multi-TeV gamma-rays). These facilities will be combined with the qualitatively new windows provided by particle astronomy via cosmic rays (Auger) and neutrinos (IceCube), and with gravitational wave astronomy with Advanced LIGO. High-energy astrophysics sources provide prime targets for these observatories and pose unique puzzles for theory.