The aim of this project is to develop a diffraction based transmission X-ray microscope, d-TXM, for non-destructive structural characterization of polycrystalline materials such as metals, ceramics, semiconductors, dust, soil and rocks, and for R&D applications in e.g. the energy-, electronics- and environmental sectors. Uniquely, d-TXM will be able to visualise the grains inside 100 micrometer thick specimens with a spatial resolution of 10-30 nm. Up to a thousand grains may be mapped simultaneously in three dimensions with respect to morphology, phase, orientation and local stress-state. Furthermore, the method will be sufficiently fast to enable the acquisition of 3D movies of the time evolution of the structure in nano-materials and components during synthesis, processing or operation.During the last decade the applicant pioneered and matured a set of X-ray based methods for 3D studies of polycrystals on the micrometre scale. For this achievement, he is recognized as a worldwide leading figure in X-ray instrumentation for structural materials, situated at a nodal point between materials, X-ray physics, applied mathematics and crystallography. The underlying vision of d-TXM is similar to this past work, but in terms of optics the microscopy approach is radically different and the spatial resolution will be two orders of magnitude better.In this project, the scientific potential will be demonstrated by means of applications to selected issues in metallurgy. Being able to directly observe the evolution of the individual crystalline elements, our understanding of processes such as plasticity and phase evolution can be greatly enhanced.Dissemination to other fields will take place via an advisory board of future users and a workshop. Continuity of the project is ensured by the technique being implemented at the European Synchrotron Research Facility.