BACKGROUND: Seasonal affective disorder (SAD) is a seasonal pattern of recurrent major depressive episodes that most commonly occurs during autumn or winter and remits in spring. The prevalence of SAD ranges from 1.5% to 9%, depending on latitude. The predictable seasonal aspect of SAD provides a promising opportunity for prevention. This review - one of four reviews on efficacy and safety of interventions to prevent SAD - focuses on light therapy as a preventive intervention. Light therapy is a non-pharmacological treatment that exposes people to artificial light. Mode of delivery (e.g. visors, light boxes) and form of light (e.g. bright white light) vary. OBJECTIVES: To assess the efficacy and safety of light therapy (in comparison with no treatment, other types of light therapy, second-generation antidepressants, melatonin, agomelatine, psychological therapies, lifestyle interventions and negative ion generators) in preventing SAD and improving patient-centred outcomes among adults with a history of SAD. SEARCH METHODS: A search of the Specialised Register of the Cochrane Depression, Anxiety and Neuorosis Review Group (CCDANCTR) included all years to 11 August 2015. The CCDANCTR contains reports of relevant randomised controlled trials derived from EMBASE (1974 to date), MEDLINE (1950 to date), PsycINFO (1967 to date) and the Cochrane Central Register of Controlled Trails (CENTRAL). Furthermore, we searched the Cumulative Index to Nursing and Allied Health Literature (CINAHL), Web of Knowledge, The Cochrane Library and the Allied and Complementary Medicine Database (AMED) (to 26 May 2014). We also conducted a grey literature search and handsearched the reference lists of all included studies and pertinent review articles. SELECTION CRITERIA: For efficacy, we included randomised controlled trials on adults with a history of winter-type SAD who were free of symptoms at the beginning of the study. For adverse events, we also intended to include non-randomised studies. We intended to include studies that compared any type of light therapy (e.g. bright white light, administered by visors or light boxes, infrared light, dawn stimulation) versus no treatment/placebo, second-generation antidepressants (SGAs), psychological therapies, melatonin, agomelatine, lifestyle changes, negative ion generators or another of the aforementioned light therapies. We also planned to include studies that looked at light therapy in combination with any comparator intervention and compared this with the same comparator intervention as monotherapy. DATA COLLECTION AND ANALYSIS: Two review authors screened abstracts and full-text publications against the inclusion criteria. Two review authors independently abstracted data and assessed risk of bias of included studies. MAIN RESULTS: We identified 2986 citations after de-duplication of search results. We excluded 2895 records during title and abstract review. We assessed 91 full-text papers for inclusion in the review, but only one study providing data from 46 people met our eligibility criteria. The included randomised controlled trial (RCT) had methodological limitations. We rated it as having high risk of performance and detection bias because of lack of blinding, and as having high risk of attrition bias because study authors did not report reasons for dropouts and did not integrate data from dropouts into the analysis.The included RCT compared preventive use of bright white light (2500 lux via visors), infrared light (0.18 lux via visors) and no light treatment. Overall, both forms of preventive light therapy reduced the incidence of SAD numerically compared with no light therapy. In all, 43% (6/14) of participants in the bright light group developed SAD, as well as 33% (5/15) in the infrared light group and 67% (6/9) in the non-treatment group. Bright light therapy reduced the risk of SAD incidence by 36%; however, the 95% confidence interval (CI) was very broad and included both possible effect sizes in favour of bright light therapy and those in favour of no light therapy (risk ratio (RR) 0.64, 95% CI 0.30 to 1.38). Infrared light reduced the risk of SAD by 50% compared with no light therapy, but in this case also the CI was too broad to allow precise estimations of effect size (RR 0.50, 95% CI 0.21 to 1.17). Comparison of both forms of preventive light therapy versus each other yielded similar rates of incidence of depressive episodes in both groups (RR 1.29, 95% CI 0.50 to 3.28). The quality of evidence for all outcomes was very low. Reasons for downgrading evidence quality included high risk of bias of the included study, imprecision and other limitations, such as self rating of outcomes, lack of checking of compliance throughout the study duration and insufficient reporting of participant characteristics.Investigators provided no information on adverse events. We could find no studies that compared light therapy versus other interventions of interest such as SGA, psychological therapies, melatonin or agomelatine. AUTHORS' CONCLUSIONS: Evidence on light therapy as preventive treatment for patients with a history of SAD is limited. Methodological limitations and the small sample size of the only available study have precluded review author conclusions on effects of light therapy for SAD. Given that comparative evidence for light therapy versus other preventive options is limited, the decision for or against initiating preventive treatment of SAD and the treatment selected should be strongly based on patient preferences.