The sound speeds of solar models that include element diffusion agree with helioseismological measurements to a rms discrepancy of better than 0.2% throughout almost the entire sun. Models that do not include diffusion, or in which the interior of the sun is assumed to be significantly mixed, are effectively ruled out by helioseismology. Standard solar models predict the measured properties of the sun more accurately than is required for applications involving solar neutrinos.

4 pages, latex file. Revised numbers for Table 1. To appear in Physical Review Letters