  We consider lattice walks in the plane starting at the origin, remaining in
the first quadrant and made of West, South and North-East steps. In 1965,
Germain Kreweras discovered a remarkably simple formula giving the number of
these walks (with prescribed length and endpoint). Kreweras' proof was very
involved and several alternative derivations have been proposed since then. But
the elegant simplicity of the counting formula remained unexplained. We give
the first purely combinatorial explanation of this formula. Our approach is
based on a bijection between Kreweras walks and triangulations with a
distinguished spanning tree. We obtain simultaneously a bijective way of
counting loopless triangulations.
