Valerian is an herb that is widely available in a variety of commercial preparations and is commonly used as a sleep aid. A recent systematic review and meta-analysis of valerian concluded that evidence in support of the effectiveness of the herb was inconclusive. Therefore, in an effort to more closely examine this issue, a systematic review was conducted to examine the evidence on the efficacy of valerian as a sleep aid with specific attention to the type of preparations tested and the characteristics of the subjects studied. A comprehensive search of studies investigating valerian was conducted through computerized databases and hand searches of reference lists. Standardized forms were used to summarize findings and standardized criteria were used to assess study quality. Out of 592 articles initially identified, a total of 36 articles describing 37 separate studies met criteria for review: 29 controlled trials evaluated for both efficacy and safety, and eight open-label trials evaluated for safety only. Most studies found no significant differences between valerian and placebo either in healthy individuals or in persons with general sleep disturbance or insomnia. None of the most recent studies, which were also the most methodologically rigorous, found significant effects of valerian on sleep. Overall, the evidence, while supporting that valerian is a safe herb associated with only rare adverse events, does not support the clinical efficacy of valerian as a sleep aid for insomnia.