  Cosmic explosions dissipate energy into their surroundings on a very wide
range of time-scales: producing shock waves and associated particle
acceleration. The historical culprits for the acceleration of the bulk of
Galactic cosmic rays are supernova remnants: explosions on ~10000 year
time-scales. Increasingly however, time-variable emission points to rapid and
efficient particle acceleration in a range of different astrophysical systems.
Gamma-ray bursts have the shortest time-scales, with inferred bulk Lorentz
factors of ~1000 and photons emitted beyond 100 GeV, but active galaxies,
pulsar wind nebulae and colliding stellar winds are all now associated with
time-variable emission at ~TeV energies. Cosmic photons and neutrinos at these
energies offer a powerful probe of the underlying physical mechanisms of cosmic
explosions, and a tool for exploring fundamental physics with these systems.
Here we discuss the motivations for high-energy observations of transients, the
current experimental situation, and the prospects for the next decade, with
particular reference to the major next-generation high-energy observatory CTA.
