  The precision expected for the rare $K\to \pi \nu\bar\nu$ decays by the NA62
and KOTO experiments in the coming decade will rival their current SM
predictions. In preparation for this upcoming opportunity, we review the SM
predictions and discuss the sensitivity of these decays to models beyond the
Standard Model, considering in particular simplified Z and Z' models as
benchmarks. In the latter case we also discuss how these decays could
ultimately probe distance scales as small as zeptometers i.e. peek into the
Zeptouniverse.
