  The INTEGRAL mission has played a major role in blazar science, thanks to its
sensitive coverage of a spectral region (3-100 keV) that is critical for this
type of sources, to its flexibility of scheduling and to the large field of
view of its cameras. A number of flat-spectrum radio quasars (up to z ~ 3) and
BL Lac objects were observed by INTEGRAL together with facilities at all
wavelengths. These results have advanced our knowledge of blazars from a
physical and cosmological point of view. This paper reviews some of these
outcomes, with particular reference to the INTEGRAL program for blazars in
outburst as targets of opportunity, with a perspective into a future of
multi-messenger astronomy
