This book presents for the first time, with a broad and unifying perspective, the common aspects and differences of the physics of the interaction between masses (gravitation), charges (Coulomb or dipolar interactions) but also plasma (from astrophysical context to charged beams) and two-dimensional hydrodynamics. The first part describes the theoretical and computational methods needed to address the study of equilibrium of these systems when considering a largenumber of particles. The second part is devoted to the presentation of the dynamical description of their out-of-equilibrium features. Finally, the third part focuses successively on the differentdomains of applications. The book, intended for Master and PhD students, tries to gradually acquaint the reader with the subject and the only prerequisite is a basic course in statistical mechanics.