Formal Power Series and Linear Systems of Meromorphic Ordinary Differential Equations Simple Ordinary Differential Equations may have solutions in terms of power series whose coefficients grow at such a rate that the series has a radius of convergence equal to zero. In fact every linear meromorphic system has a formal solution of a certain form which can be relatively easily computed but which generally involves such power series diverging everywhere. In this book the author presents the classical theory of meromorphic systems of ODE in the new light shed upon it by the recent achievements in the theory of summability of formal power series. Basic Properties of Solutions Singularities of First Kind Highest Level Formal Solutions Asymptotic Power Series Integral Operators Summable Power Series Cauchy Heine Transform Solutions of Highest Level Stokes’ Phenomenon Multisummable Power Series Ecalle’s Acceleration Operators Other Related Questions Applications in Other Areas and Computer Algebra Some Historical Remarks.