  In this talk I give a theoretical overview of the rare decays $B_{s} \to
\mu^+ \mu^-$ and $B_{d} \to \mu^+ \mu^-$. The branching ratios of these decays
are promising probes of New Physics, both independently and relative to each
other. Recent experimental progress at the LHC has confirmed the existence of
the $B_{s} \to \mu^+ \mu^-$ decay, and has not revealed any large signals of
New Physics that may have been present. This raises the question of whether
moderate New Physics effects can be identified in the LHC era. To that end I
review several important developments in the Standard Model branching ratio
predictions, and discuss how the latest measurements currently constrain New
Physics. Furthermore, I highlight how a time-dependent analysis of $B_{s} \to
\mu^+ \mu^-$, which may be feasible at the upgraded CMS and LHCb detectors, can
complement the search for and identification of New Physics.

High Energy Physics - Phenomenology