  Given a solution $u$ to a linear homogeneous second order elliptic equation
with Lipschitz coefficients, we introduce techniques for giving improved
estimates of the critical set $\Cr(u)\equiv \{x:|\nabla u|(x)=0\}$. The results
are new even for harmonic functions on $\dR^n$. Given such a $u$, the standard
{\it first order} stratification $\{\cS^k\}$ of $u$ separates points $x$ based
on the degrees of symmetry of the leading order polynomial of $u-u(x)$. In this
paper we give a quantitative stratification $\{\cS^k_{\eta,r}\}$ of $u$, which
separates points based on the number of {\it almost} symmetries of {\it
approximate} leading order polynomials of $u$ at various scales. We prove
effective estimates on the volume of the tubular neighborhood of each
$\cS^k_{\eta,r}$, which lead directly to $(n-2+\epsilon)$-Minkowski content
estimates for the critical set of $u$. With some additional regularity
assumptions on the coefficients of the equation, we refine the estimate to a
uniform $(n-2)$-Hausdorff measure estimate on the critical set of $u$.
