This book presents correlated systems in one dimension. Recent progress in nanotechnology and material research has made one-dimensional systems a crucial part of today's physics. After an introduction to the basic concepts of correlated systems, the book gives a step-by-step description of the techniques needed to treat one-dimension systems, and discusses the resulting physics. Then, specific experimental realizations of one-dimensional systems such as spin chains, quantum wires, nanotubes, and organic superconductors are examined.