BACKGROUND Dental sealants were introduced in the 1960s to help prevent dental caries in the pits and fissures of mainly the occlusal tooth surfaces. Sealants act to prevent the growth of bacteria that can lead to dental decay. There is evidence to suggest that fissure sealants are effective in preventing caries in children and adolescents when compared to no sealants. Their effectiveness may be related to the caries prevalence in the population. OBJECTIVE To compare the effects of different types of fissure sealants in preventing caries in permanent teeth in children and adolescents. METHODS We searched the Cochrane Oral Health Group's Trials Register (to 1 November 2012); the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2012, Issue 7); MEDLINE via OVID (1946 to 1 November 2012); EMBASE via OVID (1980 to 1 November 2012); SCISEARCH, CAplus, INSPEC, NTIS and PASCAL via STN Easy (to 1 September 2012); and DARE, NHS EED and HTA (via the CAIRS web interface to 29 March 2012 and thereafter via Metaxis interface to September 2012). There were no language or publication restrictions. We also searched for ongoing trials via ClinicalTrials.gov (to 23 July 2012). METHODS Randomised or quasi-randomised controlled trials of at least 12 months duration comparing sealants for preventing caries of occlusal or approximal surfaces of premolar or molar teeth with no sealant or different type of sealant in children and adolescents under 20 years of age. METHODS Two review authors independently screened search results, extracted data and assessed trial quality. We calculated the odds ratio (OR) for caries or no caries on occlusal surfaces of permanent molar teeth. For trials with a split-mouth design, the Becker-Balagtas odds ratio was used. For mean caries increment we used the mean difference. All measures are presented with 95% confidence intervals (CI). The quality of the evidence was assessed using GRADE methods. We conducted the meta-analyses using a random-effects model for those comparisons where there were more than three trials in the same comparison, otherwise the fixed-effect model was used. RESULTS Thirty-four trials are included in the review. Twelve trials evaluated the effects of sealant compared with no sealant (2575 participants) (one of those 12 trials stated only number of tooth pairs); 21 trials evaluated one type of sealant compared with another (3202 participants); and one trial evaluated two different types of sealant and no sealant (752 participants). Children were aged from 5 to 16 years. Trials rarely reported the background exposure to fluoride of the trial participants or the baseline caries prevalence.- Resin-based sealant compared with no sealant: Compared to control without sealant, second or third or fourth generation resin-based sealants prevented caries in first permanent molars in children aged 5 to 10 years (at 2 years of follow-up odds ratio (OR) 0.12, 95% confidence interval (CI) 0.07 to 0.19, six trials (five published in the 1970s and one in 2012), at low risk of bias, 1259 children randomised, 1066 children evaluated, moderate quality evidence). If we were to assume that 40% of the control tooth surfaces were decayed during 2 years of follow-up (400 carious teeth per 1000), then applying a resin-based sealant will reduce the proportion of the carious surfaces to 6.25% (95% CI 3.84% to 9.63%); similarly if we were to assume that 70% of the control tooth surfaces were decayed  (700 carious teeth per 1000), then applying a resin-based sealant will reduce the proportion of the carious surfaces to 18.92% (95% CI 12.28% to 27.18%). This caries preventive effect was maintained at longer follow-up but both the quality and quantity of the evidence was reduced (e.g. at 48 to 54 months of follow-up OR 0.21, 95% CI 0.16 to 0.28, four trials (two studies at low risk of bias and two studies at high risk of bias), 482 children evaluated; risk ratio (RR) 0.24, 95% CI 0.12 to 0.45, one study at unclear risk of bias, 203 children evaluated).- Glass ionomer sealant compared with no sealant: There is insufficient evidence to make any conclusions about whether glass ionomer sealants, prevent caries compared to no sealant at 24-month follow-up (mean difference in DFS -0.18, 95% CI -0.39 to 0.03, one trial at unclear risk of bias, 452 children randomised, 404 children evaluated, very low quality evidence).- Sealant compared with another sealant: The relative effectiveness of different types of sealants remained inconclusive in this review. Twenty-one trials directly compared two different sealant materials. Several different comparisons were made according to type of sealant, outcome measure and duration of follow-up. There was great variation with regard to comparisons, outcomes, time of outcomes reported and background fluoride exposure if this was reported.Fifteen trials compared glass ionomer with resin sealants and there is insufficient evidence to make any conclusions about the superiority of either of the two materials. Although there were 15 trials the event rate was very low in many of these which restricted their contribution to the results.Three trials compared resin-modified glass ionomer with resin sealant and reported inconsistent results.Two small low quality trials compared polyacid-modified resin sealants with resin sealants and found no difference in caries after 2 years.- Adverse effects: Only two trials mentioned adverse effects and stated that no adverse effects were reported by participants. CONCLUSIONS The application of sealants is a recommended procedure to prevent or control caries. Sealing the occlusal surfaces of permanent molars in children and adolescents reduces caries up to 48 months when compared to no sealant, after longer follow-up the quantity and quality of the evidence is reduced. The review revealed that sealants are effective in high risk children but information on the magnitude of the benefit of sealing in other conditions is scarce. The relative effectiveness of different types of sealants has yet to be established.