Many clinicians, medical practitioners and decision makers have no formal training In epidemiology but need to understand and sometimes evaluate results in epidemiologic studies. This paper attempts to give guidance to non-epidemiologists on how to read and evaluate the quality of epidemiologic studies and their results critically. Different methodological issues for evaluating whether the results of a study are causal or by bias, chance or confounding are given. This includes criteria for the choice of an appropriate study design followed by problems of the definition of the study population and the sample selection. We will also point to potential sources of bias in the data collection procedure and list some principles for statistical analysis. Finally, we include comments on how the results should be presented and issues which are related to public health and ethical questions. Although it is not usually possible to perform a perfect study and the correct approach to study design and analysis is often highly dependent on specific features of the population and the surroundings, our paper should help to distinguish between weak and strong investigations and papers.