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Multiwavelength and Multimessenger Observations of Blazars and Theoretical Modeling: Blazars as Astrophysical Neutrino Sources

This contribution reviews recent advances in the possible identification of blazars as potential sources of at least some of the very-high-energy neutrinos detected by the IceCube neutrino detector at the South Pole. The basic physical requirements for neutrino production and physics constraints that may be drawn from neutrino - blazar associations are reviewed. Several individual cases of possible associations will be discussed in more detail. It is emphasized that due to $γγ$ opacity constraints in efficiently neutrino-producing blazars, an association between X-ray -- soft $γ$-ray activity and very-high-energy neutrino production is more naturally expected than a connection between neutrino and high-energy/very-high-energy $γ$-ray activity.