  Observations over the last two decades have shown that a significant fraction
of all flat-spectrum, extragalactic radio sources exhibit flux density
variations on timescales of a day or less at frequencies of several GHz. It has
been demonstrated that interstellar scintillation (ISS) is the principal cause
of such rapid variability. Observations of ISS can be used to probe very
compact, microarcsecond-scale structure in quasar inner jets, as well as
properties of turbulence in the local Galactic ISM. A few sources show
unusually rapid, intra-hour variations, evidently due to scattering in very
nearby, localized turbulence. We present recent findings for the rapidly
scintillating quasar PKS 1257-326. The large-scale MASIV VLA Survey showed that
such sources are extremely rare, implying that for most scintillating sources,
longer-term, dedicated monitoring programs are required to extract detailed
information on source structures.
