Description Abstract: It is not yet evident that a single type of cosmic accelerator can be responsible for the Ultra-High Energy Cosmic Ray (UHECR) and the high-energy neutrinos skies at the same time. It is important to address this question, since only UHECR observations seem insufficient for the discovery of these most violent cosmic accelerators. The possibility to perform "UHECR astronomy" is nowadays discouraging after measurements by the Pierre Auger Observatory indicated a light nuclear mass composition. For theory, this key observation implies that modern source radiation models need to include nuclei, which can disintegrate on dense radiation fields of the source and initiate cascades of secondary isotopes. This talk aims to convey the basic principles of multi-messenger approaches and currently favoured astrophysical source models, demonstrating that none of the promising candidates, like Gamma-Ray Bursts or Blazars, can be excluded at this point.