In cognitive neuroscience, brain activity is typically associated with certain cognitive functions, such as decision making or working memory. However, the majority of brain activity is not spent on such cognitive functions, but generated spontaneously. The brain devotes more than 95% of its energy consumption towards spontaneous activity, pointing towards a critical role for brain function. I study spontaneous brain activity using a variety of techniques and species, ranging from fMRI in humans, and in vivo electrophysiology in cats, rodents and primates, to computational modelling. In my talk, I’ll explore various aspects of spontaneous cortical activity: its spatial structure, its interaction with stimulus-evoked responses, and its significance for cognition and behaviour. Taken together, these studies show that spontaneous brain activity plays an important role in sensory processing, and argue that understanding the brain’s internal dynamics is essential to understanding the brain as a whole.