Neurological disorders are becoming increasingly common in our aging society. In order to improve our understanding of the brain and find therapy and treatment, methods are required to precisely control and monitor neuronal signals. Optogenetics and fluorescence imaging use light for stimulation and recording, respectively. This enables unprecedented spatial and temporal control over neuronal activity and read-out from thousands of cells simultaneously. The use of optics for interrogation and monitoring requires light sources and sensors that can be integrated with soft tissue and operated in direct contact with cells. In my talk, I will present how organic semiconductor devices, which are based on plastic-type materials that are fabricated on micrometre-thin, flexible substrates, may be used as brain interfaces. Multi-coloured organic LEDs are used in vitro and for activation and inhibition of neuronal activity in Drosophila melanogaster (fruit flies). Furthermore, the development of flexible sensors for fluorescence imaging is discussed and first results using organic LEDs and photodiodes will be presented.