Over the last decade many studies were conducted to assess the feasibility of early detection of people at risk of developing psychosis and intervention to prevent or delay a first psychotic episode. Most of these studies were small and underpowered. A meta-analysis can demonstrate the effectiveness of the efforts to prevent or postpone a first episode of psychosis. A search conducted according the PRISMA guideline identified 10 studies reporting 12-month follow-up data on transition to psychosis, and 5 studies with follow-ups varying from 24 to 48 months. Both random and fixed effects meta-analyses were conducted. The quality of the studies varied from poor to excellent. Overall the risk reduction at 12 months was 54% (RR = 0.463; 95% CI = 0.33–0.64) with a Number Needed to Treat (NNT) of 9 (95% CI = 6–15). Although the interventions differed, there was only mild heterogeneity and publication bias was small. All subanalyses demonstrated effectiveness. Also 24 to 48-month follow-ups were associated with a risk reduction of 37% (RR = .635; 95% CI = 0.44–0.92) and a NNT of 12 (95% CI = 7–59). Sensitivity analysis excluding the methodologically weakest study showed that the findings were robust. Early detection and intervention in people at ultra-high risk of developing psychosis can be successful to prevent or delay a first psychosis. Antipsychotic medication showed efficacy, but more trials are needed. Omega-3 fatty acid needs replication. Integrated psychological interventions need replication with more methodologically sound studies. The findings regarding CBT appear robust, but the 95% confidence interval is still wide.