  We describe the global structure of holomorphic webs in codimension one, and
in particular their singularity (caustic). Various concepts are introduced,
which have no interest locally near a regular point, such as the type, the
reducibility, the quasi-smoothness, the CI property (complete intersection),
the dicriticity... We prove for instance that the algebraicity of a web
globally defined on a complex projective space may be readen on its caustic
(dicriticity), at least if each irreducible component is CI, and the web
quasi-smooth. .
