We propose an  ISO target of Opportunity observation of the BL Lacartae 
object BL Lac, which is currently undergoing a major outburst at optical
energies. According to many current theories of the emission processes in 
blazars, the far-IR continuum provides the seed photons for inverse Compton 
X- and gamma rays, so a gamma-ray outburst should be accompanied (and preceeded)
by a optical and far-IR outburst, and (ii) in the inhomogeneous jet models, an
 outburst starts at optical/UV wavelengths and propagates to the longer IR 
wavelengths on timescales of days to weeks.Since the launch of ISO, only one 
optical blazar outburst (3C279) has been observed by ISO, and that in PV phase
where  quality of the ISO data are less than optimum.  This may represent the 
only chance with ISO to investigate any far IR/optical correlation in flaring 
blazars.

An accurate measurement of the IR continuum during the current outburst will 
yield major diagnostics of the physical processes in the emission region of 
these enigmatic objects.