  The CERN NA62 experiment uses a novel "kaon decay-in-flight" technique to
observe $K^+ \to \pi^+ \nu \overline{\nu}$. The preliminary result based on the
analysis of the full 2016 dataset will be presented. In agreement with the
Standard Model prediction, one candidate was observed. Under the background
hypothesis, an upper limit of $14 \times 10^{-10}$ at 95 % C.L. was placed on
the branching ratio.
