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 $|V_{cb}| = 0.0379 \pm 0.0020({\rm stat})
\pm 0.0025({\rm syst}) \pm 0.0011({\rm theory})$.