  Interstellar scintillation can be used to probe transverse sizes of radio
sources on scales inaccessible to the nominal resolution of any terrestrial
telescope, e.g. $\lesssim 10^{-6}$ arc sec. Methodology is presented that
exploits this superresolution phenomenon for both single aperture and
interferometer observations. The treatment applies to the saturated
(strong-scattering) regime and holds for both thin screens and extended media.
A general signal model for radio sources is presented, {\it scintillated
amplitude modulated noise}, which applies to compact, incoherent synchrotron
sources such as AGNs and gamma-ray burst sources and also to known, coherent
sources such as masers and pulsars. The exact probability density function for
measured intensities and interferometric visibilities is obtained by solving a
general Fredholm problem. An approximate density function is also obtained by
using the equivalent number of degrees of freedom in scintillation modulations.
The scintillation modulation variance is presented, which includes the effects
of source structure and time-bandwidth averaging in the signal processing. Two
Bayesian methods are outlined for inferring the sizes of emission regions that
use first order statistics of the intensity and visibility. Intensity
cross-correlation methods for inferring source sizes are also given. Intensity
interferometry in the the radio context is compared to the optical intensity
interferometry of Hanbury-Brown and Twiss.
