Cognition is a broad term that refers to all mental activities and encompasses attention, perception, memory, reasoning, beliefs, attributions and expectations. The aim of the present paper is to draw together the major research findings relating to the importance of cognition in insomnia. Although the research to date has tended to focus on the role of unwanted intrusive thought (also known as worry or cognitive arousal), there is evidence that a broad range of cognitive processes are important for a full understanding of insomnia. These include beliefs, attributions, expectations, perception and attention. The treatment implications of this evidence are discussed, as are priorities for future research.