  We discuss the degree to which radio propagation measurements diagnose
conditions in the ionized gas of the interstellar medium (ISM). The "signal
generators" of the radio waves of interest are extragalactic radio sources
(quasars and radio galaxies), as well as Galactic sources, primarily pulsars.
The polarized synchrotron radiation of the Galactic non-thermal radiation also
serves to probe the ISM, including space between the emitting regions and the
solar system. Radio propagation measurements provide unique information on
turbulence in the ISM as well as the mean plasma properties such as density and
magnetic field strength. Radio propagation observations can provide input to
the major contemporary questions on the nature of ISM turbulence, such as its
dissipation mechanisms and the processes responsible for generating the
turbulence on large spatial scales. Measurements of the large scale Galactic
magnetic field via Faraday rotation provide unique observational input to
theories of the generation of the Galactic field.
