Juergen Fleig works in the field of Solid State Ionics with a main emphasis on the defect chemistry of oxides and their application to materials for energy conversion technologies such as solid oxide cells. Among others, he aims to understand the mechanisms behind electrochemical reactions at gas solid interfaces, such as oxygen reduction and evolution. The group pioneered the development of advanced concepts for the physico-chemical description of such reactions. A further research topic is the in-depth understanding of ion transport in solids with emphasis on conduction processes accompanied by compositional changes, i.e. under polarization, e.g. in battery electrodes. An important role in all these studies is played by the continuous development of novel methods and tools as well as novel models for the analysis and understanding of the experiments.