Our main research interests are the physical principles and molecular mechanisms determining biomolecular function. A particular focus are intrinsically disordered proteins (IDPs). At variance with the classical structure-function paradigm that we are familiar with from 50 years of structural biology, IDPs lack a persistent structure but, nevertheless, participate in many biological processes such as transcription, translation and signal transduction. In our lab, we combine fluorescence single-molecule spectroscopy with concepts from polymer physics to explain fundamental properties of disordered proteins, such as the role of electrostatic interactions, ion screening and macromolecular crowding on their conformations, as well as the contribution of internal friction to their dynamics. We are particularly interested in developing new tools to investigate the role of disordered proteins in intracellular phase transitions.