Let S be an oriented surface of genus g and n punctures. The periods of any meromorphic differential on S, with respect to a choice of complex structure, determine a representation χ : Γg,n → C where Γg,n is the first homology group of S. We characterize the representations that thus arise, that is, lie in the image of the period map Per : ΩMg,n → Hom(Γg,n, C). This generalizes a classical result of Haupt in the holomorphic case. Moreover, we determine the image of this period map when restricted to any stratum of meromorphic differentials, having prescribed orders of zeros and poles. Our proofs are geometric, as they aim to construct a translation structure on S with the prescribed holonomy χ. Along the way, we describe a connection with the Hurwitz problem concerning the existence of branched covers with prescribed branching data. Contents 1. Introduction 1 2. Geometry of translation surfaces 7 3. Surgeries on translation surfaces 9 Part I. Translation structures with prescribed holonomy 11 4. Some preliminaries 12 5. How to geometrize handles 12 6. How to geometrize punctured spheres 15 7. Geometrizing open surfaces 17 Part II. Translation structures in a prescribed stratum 22 8. Trivial holonomy I: Punctured Spheres 22 9. Trivial holonomy II: Positive genus surfaces 28 10. Spheres with non-trivial holonomy 35 11. Positive genus surfaces with non-trivial holonomy 38 12. The case of rational holonomy 44 Appendix A. Periods of meromorphic differentials with prescribed poles 54 Appendix B. Proof of Lemma 10.3 55 References 57