Arithmetic and Geometry Around Galois Theory Preface J. Bertin: Algebraic stacks with a view toward moduli stacks of covers M. Romagny: Models of curves A. Cadoret: Galois categories: M. Emsalem. Fundamental groupoid scheme N. Borne: Extension of Galois groups by solvable groups and application to fundamental groups of curves M.A. Garuti: On the “Galois closure” for finite morphisms J. C. Douai: Hasse Principle and Cohomology of Groups Z. Wojtkowiak: Periods of mixed Tate motives examples l adic side L. Bary Soroker and E. Paran: On totally ramified extensions of discrete valued fields R. P. Holzapfel and M. Petkova: An Octahedral Galois Reflection Tower of Picard Modular Congruence Subgroups. This Lecture Notes volume is the fruit of two research level summer schools jointly organized by the GTEM node at Lille University and the team of Galatasaray University (Istanbul): Geometry and Arithmetic of Moduli Spaces of Coverings (2008) and Geometry and Arithmetic around Galois Theory (2009) . The volume focuses on geometric methods in Galois theory. The choice of the editors is to provide a complete and comprehensive account of modern points of view on Galois theory and related moduli problems using stacks gerbes and groupoids. It contains lecture notes on étale fundamental group and fundamental group scheme and moduli stacks of curves and covers. Research articles complete the collection.