  The study of CP violation in $B_s^0$ oscillations is one of the key goals of
the LHCb experiment. Effects are predicted to be very small in the Standard
Model but can be significantly enhanced in many models of new physics. We
present the world's best measurement of the CP-violating phase $\phi_s$ using
$B_s^0\to J/\psi KK$ and $B_s^0\to J/\psi\pi\pi$ decays.
