For the first time this subject, including many systems of interest in Condensed Matter Physics, is treated in an unified way. Complexity emerges as one of the main ingredients dictating the collective behaviour of many systems. Glassy systems constitute one of the most interesting fields of Condensed Matter Physics for which also a considerable amount of experimental data and industial applications have been collected during the last twenty years. Systems exhibiting glassy behaviour are for example: real glasses, spin glasses, vortex flasses in superconductors, protein folding, etc. In this book the reader can see how the present theoretical understanding of these subjects is based on similar techniques and approaches hopefully allowing to develop a unifying structure that underlies the physical mechanism.