  The first observation of the decay $K^+ \to \pi^+ \nu \bar{\nu}$ (pnn) has
been reported. The E787 experiment presented evidence for this decay, based on
the observation of a single clean event from data collected during the 1995 run
of the AGS (Alternating Gradient Synchrotron at Brookhaven National
Laboratory). The branching ratio indicated by this observation, B(pnn) =
4.2$^{+9.7}_{-3.5}\times10^{-10}$, is consistent with the Standard Model
expectation although the central experimental value is four times larger. The
final E787 data sample, from the 1995--98 runs, should reach a sensitivity of
about five times that of the 1995 run alone. A new experiment, E949, has been
given scientific approval and should start data collection in 2001. It is
expected to achieve a sensitivity of more than an order of magnitude below the
prediction of the Standard Model.
