Resolution of Singularities of Vector Fields in Dimension Three

Let X be an analytic vector field defined in a real analytic manifold of dimension three. We prove that all the singularities of X can be made elementary by a finite number of blowing-ups in the ambient space. New version: Some misprints have been corrected and the text have been slightly reorganized. The final version appears in Acta Mathematica -Volume 197, Number 2, 167-289.