  VERITAS is an array of four imaging atmospheric Cherenkov telescopes that
observes gamma-ray sources in the very high energy range (VHE, E>100 GeV). A
large fraction of the known gamma-ray sources are of extragalactic origin and
belong to the blazar class: active galactic nuclei with relativistic jets
pointing along the line of sight. Gamma-ray emission from blazars is typically
variable. Part of the VERITAS observation program is devoted to follow-up
triggers on flaring blazars, initiated by observations at other wavelengths. We
will present those target of opportunity observations based on optical and GeV
(Fermi-LAT) alerts performed with VERITAS during the past 2 seasons, and
discuss the results in a multiwavelength context.
