  We present a systematical analysis of uncertainties in the helioseismological
determination of quantities characterizing the solar structure. We discuss the
effect of errors on the measured frequencies, the residual solar model
dependence and the uncertainties of the inversion method. We find
Y_{ph}=0.238-0.259, $R_b/R_\odot=0.708-0.714$ and $\rho_b=(0.185-0.199)$
gr/cm^3 (the index b refers to the bottom of the convective envelope). In the
interval $0.2<R/R_\odot<0.65$, the quantity U=P/\rho is determined with and
accuracy of $\pm 5$\permille or better. The predictions of a few recent solar
model calculations are compared with helioseismological results.
