String Field Theory : a modern introduction Introduction Worldsheet path integral: vacuum amplitudes Worldsheet path integral: scattering amplitudes Worldsheet path integral: complex coordinates Conformal field theory in D dimensions Conformal field theory on the plane CFT systems BRST quantization String field Free BRST string field theory Introduction to off shell string theory Geometry of moduli spaces and Riemann surfaces Off shell amplitudes. Amplitude factorization and Feynman diagrams Closed string field theory Background independence Superstring Momentum space SFT. This textbook provides an introduction to string field theory (SFT). String theory is usually formulated in the worldsheet formalism which describes a single string (first quantization). While this approach is intuitive and could be pushed far due to the exceptional properties of two dimensional theories it becomes cumbersome for some questions or even fails at a more fundamental level. These motivations have led to the development of SFT a description of string theory using the field theory formalism (second quantization). As a field theory SFT provides a rigorous and constructive formulation of string theory. The main focus of the book is the construction of the closed bosonic SFT. The accent is put on providing the reader with the foundations conceptual understanding and intuition of what SFT is. After reading this book the reader is able to study the applications from the literature. The book is organized in two parts. The first part reviews the notions of the worldsheet theory that are necessary to build SFT (worldsheet path integral CFT and BRST quantization). The second part starts by introducing general concepts of SFT from the BRST quantization. Then it introduces off shell string amplitudes before providing a Feynman diagrams interpretation from which the building blocks of SFT are extracted. After constructing the closed SFT the author outlines the proofs of several important properties such as background independence unitarity and crossing symmetry. Finally the generalization to the superstring is also discussed.