What do scintillations tell us about the ionized ISM?

I review what has been learned about the fine structure in the ionized interstellar medium from radio scintillation and scattering - first, as it constrains the wavenumber spectrum of the electron density and second in terms of evidence from parabolic scintillation arcs for discrete structures on scales less than about 1 AU, which must be very widely distributed to account for the incidence of discrete parabolic arcs and Extreme Scattering Events.