The cortex constantly generates spontaneous activity that is strongly correlated to states of vigilance, cognitive processes and motor activity and is believed to affect sensory perception. The cellular correlates and mechanisms controlling cortical states remained poorly understood until recently. The development of methodologies allowing for the study of brain functions at cellular levels in awake animals, on one hand, and of tools allowing for the recording or manipulation of the neuronal activity of genetically-defined cell types, on the other hand, has led to major advances in our understanding of cortical states. Over the last decade, we have investigated how cortical states are generated in the mouse sensory cortex and how cortical states interact with sensory processing and perception.