Systematic research reviews have become essential in all empirical sciences. However, the validity of research syntheses is threatened by the fact that not all studies on a given topic can be summarized. Research reviews may suffer from missing data, and this is especially crucial in those cases where the selectivity of studies and their findings affects the summarized result. So-called publication bias is a type of missing data and a phenomenon that jeopardizes the validity of systematic or quantitative, as well as narrative, reviews. Publication bias exists if the preparation, submission or publication of research findings depend on characteristics of just these research results, e. g. their direction or statistical significance. This article describes methods to identify publication bias in the context of meta-analysis. It also reviews empirical studies on the prevalence of publication bias, especially in the social and economic sciences, where publication bias also seems to be prevalent. Several proposals to prevent publication bias are discussed.