  Intraday variability (IDV) of compact extragalactic radio sources is a
complex phenomenon and shows a wavelength dependent mixture of refractive
interstellar scintillation (RISS) (dominant at long cm-wavelengths) and
source-intrinsic effects (dominant at shorter wavelengths). Detailed
investigations of individual sources and new high frequency observations
suggest a source-intrinsic contribution to the IDV pattern at least in some
sources. However, the sizes of intraday variable sources at cm-wavelength are
typically smaller than the scattering size set by the ISM in our galaxy and
scintillation must be present, too. We present new IDV observations in
different regimes (from cm to sub-mm wavelengths) and show how such
multi-frequency study can be used as powerful instrument to describe different
aspects of IDV.
