Several studies have analyzed whether sustainability standards—such as Fairtrade or Organic—deliver on their promise to benefit smallholder farmers in developing countries, with mixed results. We conduct a systematic review and meta-analysis to consolidate results from 97 original studies. We place a focus on economic effects of sustainability standards and outcome variables that are frequently considered in quantitative studies, including output prices, yields, production costs, farmer profits, and household income. Results suggest that farmers certified under a sustainability standard receive 20–30% higher prices than their non-certified counterparts. Effects of standards on production costs and yields are mixed and vary across standards. Certified farmers gain higher profits, leading to an overall increase in household incomes through standards by 16–22%. Yet substantial heterogeneity exists, which is only partly attributable to observed factors that vary across studies (such as the type of product, standard, or region). Our findings suggest that more context-specific factors—such as the organization of supply chains—play a more decisive role. Based on a critical review of the sampling strategies and methods employed in the original studies, we discuss the generalizability of our findings and derive directions for policy and future research.