  Intraday Variability of compact extragalactic radio sources can be
interpreted as quenched scintillation due to turbulent density fluctuations of
the nearby ionized interstellar medium. We demonstrate that the statistical
analysis of IDV time series contains both information about sub-structure of
the source on the scale of several 10 micro-arcsec and about the turbulent
state of the ISM. The source structure and ISM properties cannot be
disentangled using IDV observations alone. A comparison with the known
morphology of the `local bubble' and the turbulent ISM from pulsar observations
constrains possible source models. We further argue that earth orbit synthesis
fails for non-stationary relativistic sources and no reliable 2D-Fourier
reconstruction is possible.
