The interstellar plasma exhibits a broad band spectrum of density fluctuations. These are responsible for scintillations (both diffractive and refractive), dispersion measure fluctuations, and the spreading of the angular images of compact radio sources. These reveal an electron density power spectrum which is fit by a single power law, with spectral index close to the Kolmogorov value of 5/3 over at least six orders of magnitude in spatial scale. Moreover, the 5/3 index is universal; it applies irrespective of the magnitude of the density fluctuations. The detection of elliptical, scatter-broadened images implies that the density fluctuations are anisotropic, presumably aligned with respect to the direction of the local magnetic field. The propagation of radio waves through the interstellar plasma is analogous to that of visible light through the earth's atmosphere. We know that stellar scintillations and image smearing arise from density fluctuations created by hydrodynamic, turbulent mixing of parcels of air having different specific entropy. It is thus natural to suspect that turbulence, in this case magnetohydrodynamic (MHD), may be responsible for the spectrum of interstellar plasma density fluctuations. It appears that MHD turbulence may be amenable to analysis. I will describe how nonlinear interactions among shear Alfven waves give rise to an anisotropic cascade whose velocity and magnetic perturbations conform to a 2D, Kolmogorov, power spectrum. I will also explain why the cascade of the slow mode is controlled by interactions with shear Alfven waves but not vice versa