  The Fermilab HyperCP collaboration is making precision studies of charged
hyperon and kaon decays, as well as searches for rare and forbidden hyperon and
kaon decays. We report here on measurements of the branching ratios of the
flavor-changing neutral-current decays: K+/- -> pi+/- mu+ mu-, and compare our
results to theoretical predictions. This is the first observation of the K- ->
pi- mu+ mu- decay.
