The past years have seen several Bunsen Colloquia devoted to reactivity as well as diffusivity in solids, topics that are of central importance in advanced materials research and engineering. This yeara€™s Bunsen Colloquium continues this series. It focuses on defects and diffusion in solids and on the application of new theoretical concepts. Understanding the microscopic properties of solids at the level of defects and diffusion mechanisms is critical for understanding the macroscopic properties of materials. Ever more powerful supercomputers and superior program packages now allow the complex behaviour of atoms and ions to be studied. Approximations that were previously required can now be neglected due to improved algorithms, as will be discussed at this Colloquium. Topics to be covered are, for example, the explicit consideration of phonon-electron coupling in quantum mechanical calculations for electronic conductivity and the application of time-dependent DFT methods to materialsa€™ research. We are looking forward to meeting you at the RWTH Aachen University.