Interstellar scintillation (ISS) causes a Galactic seeing problem for radio astronomy. The flux density from a very compact source appears to scintillate on a time scale that ranges from days to minutes depending on its diameter, the wavelength and Galactic path length. I will review the observed variations from various sources, including quasars, AGNs and pulsars.

An ISS interpretation of the rapid (intraday) AGN variations yields source sizes in the range 0.005 to 10 milliarcsec, often much smaller than the resolution from VLBI. However, the recognition of such variations as apparent reduces the implied brightness temperature by a factor of about a million, compared to the extreme values deduced by interpreting the variations as intrinsic. Partially correlated variations in polarized flux and angle can also be explained as ISS from sources with intrinsic polarization structure at a level of tens of microarcseconds. ISS also explains rapid variability in the afterglow of gamma-ray burst 970508 (Frail et al. Nature, 389, 261, 1997) and in many of the 150 compact sources monitored at Green Bank. New observations of the flux variations at cm wavelengths from several nearby pulsars will also be presented. These give evidence for enhanced scattering at the edge of the local interstellar bubble and place constraints on the inner scale of the interstellar turbulence spectrum.