The general principles of the NA62 experiment for measuring the $K^+ \to \pi^+ \nu \bar\nu$ branching ratio are reviewed. The goal of the experiment is to collect about 100 events of $K^+ \to \pi^+ \nu \bar\nu$ in two years of data with less than 20% background. An analysis of the performances based on detailed simulations and data from various test beams shows that the sensitivity of the experiment matches the requirements. The results from the NA62 technical run of November 2012 are also reported.