We describe the development of a cognitive therapy intervention for chronic insomnia. The therapy is based on a cognitive model which suggests that the processes that maintain insomnia include: (1) worry and rumination, (2) attentional bias and monitoring for sleep-related threat, (3) unhelpful beliefs about sleep, (4) misperception of sleep and daytime deficits and (5) the use of safety behaviors that maintain unhelpful beliefs. The aim of cognitive therapy for insomnia is to reverse all five maintaining processes during both the night and the day. In an open trial 19 patients meeting diagnostic criteria for primary insomnia were treated with cognitive therapy for insomnia. Assessments were completed pretreatment, posttreatment and at 3-, 6- and 12-month followup. The significant improvement in both nighttime and daytime impairment evident at the posttreatment assessment was retained up to the 12 month followup.