Dreams are a very remarkable experiment in psychology and neuroscience, performed every night in every sleeping person. They demonstrate that our brain, disconnected from the environment, can generate for itself a whole world of conscious experiences. Content analysis and development studies have favoured our understanding of sleep phenomenology. At the same time, studies of brain lesions, functional imaging and neuro-physiology have advanced in our knowledge of the neural basis of sleep. It is now possible to begin to integrate these two streams of inquiry to address some fundamental questions that dreams pose for cognitive neuroscience: how conscious experiences in sleep relate to underlying brain activity; Why the dreamer is largely disconnected from the environment; And if the dream is more closely related to the mental imagination or the perception.