  The neutral $B$ mesons, $B^0$ and $B_s$, can oscillate between their particle
and antiparticle states owing to flavor-changing weak interactions. In recent
years, techniques to detect these oscillations as a function of the meson's
decay time have been developed. In this article the physics of flavor
oscillations is reviewed and theoretical predictions are summarized. The many
observations that demonstrate the time-dependence of $B^0-\B0bar$ oscillations
are presented along with a combined measurement of its frequency, $\delta m_d$.
The attempts to measure the $B_s$ oscillation frequency, both directly and
indirectly, are then summarized. Finally, values for the CKM elements
$|V_{td}|$ and $|V_{ts}/V_{td}|$ are extracted.
