  Measurement of the $B^0_s$ oscillation frequency via $B^0_s$ mixing analysis
provides a powerful constraint on CKM matrix elements. This note briefly
reviews the motivation behind these analyses and describes the various steps
that go into a mixing measurement. Recent results on $B^0_s$ mixing obtained by
the CDF and D{\O}collaborations using the data samples collected at Tevtron
Collider in the period 2002 - 2005 are presented.
