Meta-analysis is defined to be ‘the statistical analysis of a large collection of analysis results from individual studies for the purpose of integrating the findings'. Since the 1980s there has been an upsurge in the application of meta-analysis to medical research. The rapid increase in the number of meta-analysis being conducted during the last decade is mainly due to a greater emphasis on evidence based medicine and the need for reliable summaries of the vast and expanding volume of clinical studies. Over the same period there have been great developments and refinements of the associated methodology of meta-analysis. When judging the reliability of the results of a meta-analysis, attention should be focused on ‘publication bias’. Publication bias is the term for what occurs whenever the research that appears in the published literature is systematically unrepresentative of the population of completed studies. This bias can provide a flaw of the result of meta-analysis. In this article, the causes and origins of publication bias are reviewed, and then the history and some findings of publication bias in medical research are presented. Several statistical methods that have been developed to detect, quantify and assess the impact of publication bias in meta-analysis are demonstrated.