This book collects together the lecture courses and seminars given at the Les Houches Summer School 2008 on Long-Range Interacting Systems. Leading scientists in different fields of mathematics and physics present their views on this fast growing and interdisciplinary field of research, byventuring upon fundamental problems of probability, transport theory, equilibrium and non-equilibrium statistical mechanics, condensed matter physics, astrophysics and cosmology, physics of plasmas, and hydrodynamics. The thermodynamic and dynamical properties of systems with long-range interactionswere poorly understood until a few years ago. Substantial progress has been made only recently by realizing that the lack of additivity induced by long-range interactions does not hinder the development of a consistent thermodynamic formalism. This book reviews the state-of-the-art developments inthis field and provides an essential background to future studies. All chapters are written from a pedagogical perspective, making the book accessible to masters and PhD students and all researchers wishing to enter this field.