  Complete two-loop QCD corrections to $b\to c$ transitions are presented in
the limit of zero recoil. Vector and axial-vector coefficients $\eta_{A,V}$ are
calculated analytically in the limit of equal beauty and charm masses, and a
series appoximation is obtained for the general mass case. $\eta_A$ is crucial
for the determination of the absolute value of the Cabibbo-Kobayashi-Maskawa
matrix element $V_{cb}$. The two-loop effects enhance the one-loop corrections
by 21\%, removing a major theoretical uncertainty in the value of $|V_{cb}|$.
Including two-loop QCD effects and previously neglected electroweak and Coulomb
corrections we find $|V_{cb}| = 0.0379 \pm 0.0020({\rm stat}) \pm 0.0025({\rm
syst}) \pm 0.0011({\rm theory})$.
