  Let $E\subset R^d$ with $H^n(E)<\infty$, where H^n stands for the
$n$-dimensional Hausdorff measure. In this paper we prove that E is
n-rectifiable if and only if the limit $$\lim_{\ve\to0}\int_{y\in E:|x-y|>\ve}
\frac{x-y}{|x-y|^{n+1}} dH^n(y)$$ exists H^n-almost everywhere in E. To prove
this result we obtain precise estimates from above and from below for the $L^2$
norm of the n-dimensional Riesz transforms on Lipschitz graphs.
