Singularités irrégulières

French text.
The letters collected in this volume concern the irregular singularities of linear differential equations: irregularity, asymptotic expansions, Stokes sheaves, Gevrey analogues, moduli problems, multisummability, Galois and wild fundamental groups, vanishing cycles, Fourier transforms. Most of these letters were written by the authors during the period 1976-1991. Four previously unpublished texts have been added to this correspondence.