Bias can arise because of the actions of primary study investigators or because of the actions of review authors, or may be unavoidable due to constraints on how research can be undertaken in practice. Where possible, assessments of risk of bias in a systematic review should be informed by evidence. This chapter summarizes some of the key evidence about bias that informs our guidance on risk-of-bias assessments in Cochrane Reviews. Many results are often available in trial reports, so review authors should think carefully about which results to assess for risk of bias. Review authors who restrict their primary analysis in this way are encouraged to perform sensitivity analyses to show how conclusions might be affected if studies at a high risk of bias were included. The chapter also addresses how source of funding and conflicts of interest of study authors may impact on study design, conduct and reporting.