Our research in the field of synthetic biology focusses on identifying new ways to control the activity of proteins and to manipulate cellular signaling processes in models of health and disease. I will be mainly presenting our recent studies in optogenetics that had the goal to replace chemical signals (e.g. a small molecule ligand or drug) by light activation (e.g. using flavin-binding LOV domains or tetrapyrrole-binding phytochromes). Our optogenetic actuators enable light-dose-dependent control of biological systems with exceptional spatial (e.g. down to micrometers) and temporal (e.g. down to milliseconds) precision in vitro, in cells and in animals. These studies focussed on receptor proteins, in particular as kinase receptors and G-protein coupled receptors (GPCRs), and yielded new molecular tools and new insights into how receptor activation ochestrates cell behaviour.