Neutrino astronomy shares with \(\upgamma \)-ray astronomy the objective of understanding the sources and mechanisms of CR acceleration. Due to their much larger interaction cross section, \(\upgamma \)-rays are easier to detect than neutrinos, and \(\upgamma \)-ray astronomy is having a fundamental importance on several topical areas of modern astrophysics and cosmology. The existence of CR sources seems to guarantee the existence of high-energy neutrino sources, in addition to those of \(\upgamma \)-rays. While \(\upgamma \)-rays can be produced both by hadronic (through \(\pi ^0\) decay) or leptonic processes (inverse Compton, bremsstrahlung), neutrinos can only be produced by hadronic processes (\(\pi ^{\pm }\) decay), Sect. 10.1.