The chapter discusses the biases that can occur in meta-analyses, particularly in the selection and location of studies. It highlights several key biases, including publication bias, where studies with significant results are more likely to be published, leading to an overestimation of treatment effects. Other biases include language bias, where studies published in English are more likely to be included, database bias, where studies from less developed countries may be overlooked due to lack of indexing, and citation bias, where supportive studies are more frequently cited. The chapter also addresses multiple publication bias, where studies with significant results are more likely to be published multiple times. Funnel plots are introduced as a graphical method to detect these biases, showing asymmetry in the plot as a sign of bias. The authors emphasize the importance of critical examination for biases in meta-analyses and suggest that registration of trials at the time of establishment can help reduce bias.The chapter discusses the biases that can occur in meta-analyses, particularly in the selection and location of studies. It highlights several key biases, including publication bias, where studies with significant results are more likely to be published, leading to an overestimation of treatment effects. Other biases include language bias, where studies published in English are more likely to be included, database bias, where studies from less developed countries may be overlooked due to lack of indexing, and citation bias, where supportive studies are more frequently cited. The chapter also addresses multiple publication bias, where studies with significant results are more likely to be published multiple times. Funnel plots are introduced as a graphical method to detect these biases, showing asymmetry in the plot as a sign of bias. The authors emphasize the importance of critical examination for biases in meta-analyses and suggest that registration of trials at the time of establishment can help reduce bias.