By introducing the superspace formalism, the methods of structure analysis of incommensurate structures have achieved in the past few years a full maturity. The superspace description is also becoming in the field of quasicrystals the main tool to approach a systematic method of structure determination of these materials. According to the program of the Workshop, these proceedings are an introduction to the formalism and practice of structure determination of modulated structures (incommensurate and commensurate) and quasiperiodic systems, mainly under the unifying framework of the superspace description. Accordingly, a large set of tutorial introductory chapters written by well-known specialists are included. The main refinement programs available for incommensurate structures are presented by their authors. The book also contains the most recent contributions from more than thirty of the participants in the Workshop, focusing on the problem of the structure analysis of these typical materials by means of diffraction methods.