The interstellar plasma is characterized by variations of electron density having a wide range of physical scales. Radiation propagating through this inhomogeneous plasma is scattered, causing the received intensity to scintillate on a variety of time scales. Observations of the radio frequency spectrum and temporal variation of scintillation give information on the turbulence spectrum of the plasma and the distribution of density irregularities throughout the galaxy. Some uses of scintillation as a means of probing the interstellar plasma will be described.