  Blazars are jetted active galactic nuclei with a jet pointing close to the
line of sight, hence enhancing their intrinsic luminosity and variability.
Monitoring these sources is essential in order to catch them flaring and
promptly organize follow-up multi-wavelength observations, which are key to
providing rich data sets used to derive e.g., the emission mechanisms at work,
and the size and location of the flaring zone. In this context, the Fermi-LAT
has proven to be an invaluable instrument, whose data are used to trigger many
follow-up observations at high and very high energies. A few examples are
illustrated here, as well as a description of different data products and
pipelines, with a focus given on FLaapLUC, a tool in use within the H.E.S.S.
collaboration.
