Acta Crystallographica Section A: Foundations of Crystallography is the leading forumfor publishing papers related to fundamental advances in all areas of crystallography inthe broadest sense. At a meeting of the Editorial Board of Acta A held at the SchlossBottstein, Villigen, near the Swiss Synchrotron source at the Paul Scherrer Institute, itwas decided to publish special themed issues on topics of interdisciplinary interests wherecrystallographic tools and techniques have played very significant roles. The field of phasetransitions is truly interdisciplinary in nature and continues to be actively pursued byphysicists, chemists, earth scientists, materials scientists and metallurgists in differentparts of the world. Accordingly it was decided to bring out one themed issue on phasetransitions containing about 20 invited topical reviews/overviews covering importantrecent developments in the field. The present issue containing nine articles is an outcomeof this initiative. It is expected that another issue of Acta A with about ten more articleswill follow the present one in the near future covering other aspects of phase transitions.The first article by Salje et al. reviews the progress made in the use of Landau-typetheories to study the ferroelastic phase transitions at three different levels, viz the crystalstructure, the microstructure and the mesostructure levels.This is followed by four articles on different aspects of phase transitions in aperiodiccrystals. Pan & Chapuis present an overview of their recent work on the simulationof experimentally measured diffraction effects due to incommensurate structures andtheir phase transitions in organic crystals using molecular dynamics techniques withgeneric force field. Steurer presents a comprehensive review of the different types ofexperimentally observed amorphous-to-quasicrystalline, quasicrystalline-to-ordered/dis-ordered-quasicrystalline and quasicrystalline-to-crystalline phase transitions as a func-tion of temperature, pressure and irradiation, as well as high-energy ball milling. Nextfollows a critical review by Sato of the progress made in the experimental understandingof magnetic quasicrystals, especially from the point of view of the nature of spin freezingat low temperatures. The next article by van Smaalen gives an overview of phase tran-sitions in low-dimensional electronic crystals towards a low-temperature charge densitywave state and the application of X-ray diffraction as a tool to obtain quantitativeinformation about such transitions.The sixth article turns towards yet another facet of phase transitions of topical interest.In this article, Moret reviews orientational ordering phase transitions and glass tran-sitions of C