Abstract: The results of a meta-analysis can be distorted if bias has been introduced into the review process at any stage at any stage (e.g. biased location and selection of included studies, high risk of bias in the results of included studies). Accordingly, investigation of the presence, degree, and nature of bias is recommended as a routine part of the systematic review process. The Bias Methods Group draws together researchers interested in this field, and the new Cochrane Handbook contains chapters on "Addressing reporting biases" and "Assessing risk of bias in included studies". The workshop will largely draw upon the work of members of this Group. We will discuss potential biases that might be introduced into reviews, with an emphasis on reporting biases and the effects of flaws in the methodology of component studies. Particular attention will be paid to publication bias (the publication or non-publication of research findings, depending on the nature and direction of the results), other reporting biases including outcome selection biases, and the empirical evidence on sources of bias in the results of randomised controlled trials. We will demonstrate and discuss graphical and statistical methods that can be used to detect or investigate bias. These include the funnel plot, "trim and fill", tests for funnel plot asymmetry, and meta-regression. The workshop will consist of Powerpoint presentations, and discussions with and among participants.