Objective: To systematically review the evidence for the effects of the herb valerian (Valeriana officinalis) on insomnia, based on randomized, placebo-controlled, double-blind trials.Background: Valerian has long been advocated and used for promoting sleep but until quite recently evidence was solely anecdotal. However, during the last two decades a number of clinical trials have been conducted.Materials and methods: Systematic literature searches were performed to locate randomized, placebo-controlled, double-blind trials measuring the effect of valerian monopreparations on sleep in human participants. Data were extracted in a standardized manner and methodological quality was assessed by the Jadad score.Results: Nine trials were located meeting the selection criteria. The findings of the studies were contradictory and there was great inconsistency between trials in terms of patients, experimental design and procedures and methodological quality.Conclusion: The evidence for valerian as a treatment for insomnia is inconclusive. There is a need for rigorous trials to determine its efficacy.