  The oscillation phenomenon in the neutral B mesons systems is now well
established.
  The motivations and principles of the measurements are given; then the most
recent results from the LEP experiments, the CDF collaboration at Fermilab and
the SLD collaboration at SLAC are reviewed.
  The present world average of the $\bd$ meson oscillation frequency is $\dmd =
0.471 \pm 0.016 \ps$ and the lower limit on the $\bs$ oscillation frequency is
$\dms > 12.4 \ps at 95% CL$.
