BACKGROUND Although pit and fissure sealants are effective in preventing caries, their efficacy may be related to the caries prevalence in the population. OBJECTIVE The primary objective of this review was to evaluate the caries prevention of pit and fissure sealants in children and adolescents. METHODS We searched the Cochrane Oral Health Group Trials Register, CENTRAL (The Cochrane Library 2007, Issue 3) and MEDLINE (to October 2007); EMBASE (to June 2007); SCISEARCH, CAplus, INSPEC, NTIS, PASCAL, DARE, NHS EED and HTA (to February 2008). There were no language or publication restrictions. METHODS Randomised or quasi-randomised controlled trials of at least 12 months in duration comparing sealants with no sealant or sealants from different classes of materials for preventing occlusal caries in children and adolescents under 20 years. The primary outcome was the increment in the numbers of carious occlusal surfaces of premolars and molars. METHODS Two review authors independently screened search results, extracted data and quality assessed trials. Risk ratios (RR) were calculated for differences between intervention and control groups and in split-mouth studies for differences of paired tooth surfaces being carious or not. The meta-analyses were conducted using a random-effects model. RESULTS Sixteen studies were included in the review; 7 studies provided data for comparison of sealant versus control without sealant and 10 studies for comparison of sealant versus sealant. Five split-mouth studies and one parallel group study with 5 to 10 year old children found a significant difference in favour of second or third generation resin-based sealants on first permanent molars, compared to a control without sealant, with a pooled RR of 0.13 (95% confidence interval (CI) 0.09 to 0.20), 0.22 (95% CI 0.15 to 0.34), 0.30 (95% CI 0.22 to 0.40), and 0.40 (95% CI 0.31 to 0.51) at 12, 24, 36 and 48-54 months follow up, respectively. Further, one of those studies with 9 years of follow up found significantly more caries in the control group compared to resin sealant group; 27% of sealed surfaces were decayed compared to 77% of surfaces without sealant.The results of the studies comparing different sealant materials were conflicting. CONCLUSIONS Sealing is a recommended procedure to prevent caries of the occlusal surfaces of permanent molars. The effectiveness of sealants is obvious at high caries risk but information on the benefits of sealing specific to different caries risks is lacking.