Eszopiclone (Lunesta), the S-enantiomer of racemic zopiclone, is a nonbenzodiazepine hypnotic agent that is approved in the US as an oral, once-nightly therapy for insomnia in adults; eszopiclone is also currently under review by the European Medicines Agency. Eszopiclone is rapidly absorbed after oral administration without any next-day clinical residual effects being detected. Large, well designed trials of up to 6 months' duration have shown that eszopiclone significantly improves both sleep onset and sleep maintenance compared with placebo in adult and elderly patients with primary insomnia. Eszopiclone for 4-8 weeks also significantly improved sleep parameters compared with placebo in patients with insomnia coexisting with other conditions that also disturb sleep (co-morbid insomnia), and improved certain measures of the co-morbid conditions to a greater extent than the standard therapies alone. Short-term eszopiclone produced improvements in daytime functioning in patients with co-morbid insomnia. Six months' therapy in adults with primary insomnia improved daytime functioning and health-related quality of life. Eszopiclone was generally well tolerated. There was no evidence of tolerance during 12 months' treatment with this agent. On discontinuation of eszopiclone, there was no rebound insomnia or serious withdrawal effects. Well designed, comparative trials with other nonbenzodiazepine hypnotics are needed to determine its relative efficacy and tolerability. A cost-utility analysis suggested that eszopiclone is cost effective for the treatment of primary insomnia in the US. Therefore, eszopiclone is a useful therapeutic option in the management of adult and elderly patients with primary or co-morbid insomnia. Unlike most other hypnotics, eszopiclone is not limited to short-term use.