The Transient Universe is abundant with sources that serve as exceptional laboratories for studying radiation processes in multiple windows, as evidenced by UHECRs, black holes in AGN and microquasars, SN1987A, Cygnus X-1, PSR 1913+16, CC-SNe and GRBs. As general relativity is becoming a genuine experimental science with the LAGEOS and Gravity Probe B experiments beyond mere redshift effects, and gravitational-wave and neutrino experiments are advancing to next generation sensitivity, this decade is expected to bring major new discoveries with an inevitable transformation of our understanding of their astronomical origin and the physics of their radiation processes.

This pursuit requires a concerted effort on effective observational strategies, theory, advanced data analysis and high-performance computing, including integration of the three different areas of electromagnetic, hadronic and gravitational radiation processes. We hope that the present book offers a useful introduction to this exciting development, for those who wish to pick up this challenge.

In the electromagnetic spectrum, radiation processes tend to extend far out, while neutrino and gravitational-wave emissions often tend to be confined to or nearby the energy source driving these emissions. The relation between these various windows of observations thereby tends to be non-trivial, also because the most relativistic sources are transient. It generally calls for time-dependent models that elucidate correlations in energies, time scales, light curves, the associated scaling relations and, possibly, normalizations.