  The rare decay B_s -> mu^+ mu^- provides an important constraint on possible
deviations from the Standard Model in b-s-l-l interactions. The present
weighted average of its branching ratio measurements amounts to (3.34 \pm 0.27)
x 10^-9, which remains in good agreement with the theoretical prediction of
(3.64 \pm 0.12) x 10^-9 within the Standard Model. In the present paper, we
review calculations that have contributed to this prediction, and discuss the
associated uncertainties.
