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The twinkling of stars is a familiar example of scintillations, due toturbulence in the Earth’s atmosphere causing fluctuations in therefractive index of the air along the line of sight. Scintillations lead totime variations in the apparent position of the source, and hence to anangular broadening on integration over an observation time. Scintillationsalso lead to fluctuations in the intensity of the source. Pointlikeastronomical radio sources such as pulsars and (the compact cores of some)quasars scintillate due to fluctuations in the electron density along the lineof sight through the interstellar medium. For quasars, low-frequency (100s ofMHz) variability over periods of years is a scintillation effect, as areprobably more rapid (as short as an hour) intensity variations at higher radiofrequencies. Unlike the twinkling of stars, which is due to weakscintillations, the scintillations of radio sources are usually strong.Important qualitative effects associated with strong scattering are multipathpropagation and a clear separation into diffractive and refractivescintillations. Quasars exhibit only refractive scintillations. Pulsars areextremely small and bright, and they vary temporally on a very short timescale, making them almost ideal sources on which to test our ideas onscintillations.Pulsars exhibit a variety of scintillation phenomena, due to both refractiveand diffractive effects, the latter seen most clearly in dynamic spectra.These data are used to model the distribution of electrons through the Galaxy,to determine the distribution of pulsar velocities, and potentially to resolvethe source region in a pulsar magnetosphere.These scintillation phenomena and their interpretation in terms of the theoryof strong scintillations are reviewed briefly. The generalisation of thetheory to include the birefringence of the plasma (Faraday effect), and itspossible implications on the interpretation of circular polarisation, are thenoutlined. An attempt to generalise the theory to describe scattering by adistribution of discrete scattering objects is also discussed briefly.

Twinkle, Twinkle Little Pulsar/Quasar