| Summary: | Interstellar scintillation (ISS) has been established as the cause of the random variations seen at centimetre wavelengths in many compact radio sources on timescales of a day or less. Observations of ISS can be used to probe structure both in the ionized insterstellar medium of the Galaxy and in the extragalactic sources themselves down to µ as scales. A few quasars have been found to show large amplitude scintillations on unusually rapid, intrahour timescales. This has been shownto be due to weak scattering in very local Galactic ‘screens’, within a few tens of parsec of the Sun. The short variability timescales allowdetailed study of the scintillation properties in relatively short observation periods with compact interferometric arrays. The three best-studied ‘intrahour variable’ quasars, PKS 0405-385, J1819+3845 and PKS 1257-326, have been instrumental in establishing ISS as the principal cause of intraday variability at centimetre wavelengths. Here we review the relevant results from observations of these three sources.
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