The Galactic Distribution of Electron Density Microstructure Inferred from Radio Scattering Observations

I first review the observables and optics of interstellar seeing associated with radio wave scattering in the interstellar medium. I then describe the Galactic distribution of electron density and its fluctuations, as inferred from a number of observables, including angular and pulse broadening, diffractive scintillations, and dispersion measures. Propects for improving the Galactic model are outlined. This revised version was published online in July 2006 with corrections to the Cover Date.