Optogenetics and photopharmacology are two modern, fast-developing fields that use light-responsive molecules as tools for scientific research and hold promises for medical interventions. The lecture course will present ‘sensors’ used to monitor specific molecular events, as well as light-controlled molecules used to manipulate the activity of specific cells within a cellular network or the activity of specific signaling pathways within a cell. Richly illustrated with examples, the course covers the principles of these approaches, their main advantages and limitations, as well as current challenges for their application in translational medicine.