Mixed electronic and ionic conductors (MIECs) are a special material class often employed for electrochemical energy conversion. By allowing transport of both ions and electrons, they are important as electrodes in Li-ion batteries or solid oxide fuel/electrolysis cells, but also used in sensors or gas permeation membranes. Additionally, MIECs are also valued for their catalytic properties at the surface. Some examples of such MIECs are (La,Sr)CoO_3-d, Li_1-xCoO₂ and SrTiO₃. As their ionic conductivity typically relies on point defects (e.g. ion vacancies, ions occupying interstitial lattice sites, or localized electronic charge carriers), understanding their properties requires detailed knowledge of the defect chemistry of MIECs. In our team, we use several tools, and also develop novel methods, to gain insight into defect concentrations, defect interactions and their energy states. These ionic defects then again have an influence on electronic properties for example by trapping electrons or holes. Further, ionic/electronic equilibria shift with temperature and gas atmosphere. Interface effects, such as space charges and kinetic effects (some ions may become immobile at low temperatures) are a further challenge in understanding MIECs and their application. Among others, recent activities deal with time dependent compositional changes in such MIECs caused by applied voltages or by illumination. Those changes may either be detrimental (degradation of electroceramic devices) or functional, as in the case of photoelectrochemical cells or switchable catalysts.