  The evidence for joint gamma and neutrino emission from the blazar TXS
0506+056 has renewed interest in blazars as neutrino sources. The detection of
neutrinos from blazars can be seen as the smoking gun for the presence of
relativistic protons in blazar's jets, and can thus help identify blazars, and
active galactic nuclei in general, as cosmic-ray accelerators. In this
contribution I first introduce blazars and blazar hadronic models, and then
present the results of the multi-messenger modeling of both the 2017
gamma-neutrino flare, and the 2014-2015 neutrino-only flare of TXS 0506+056.
For the first time it is possible to constrain blazar hadronic models using the
information from neutrino detectors, and I discuss the implications of these
neutrino observations on our understanding of the physics of relativistic jets
from super-massive black-holes.
