  The experimental measurement of positive kaons decaying to a positive pion
and a neutrino, anti-neutrino pair (pnn) is reviewed. Recent results from
experiment E787 at BNL are presented: with data from 1995--97 the branching
ratio has been measured to be B(pnn) = (1.5^{+3.4}_{-1.2}) \times10^{-10}. The
future prospects for additional data in this mode are examined.
