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Background: Despite the effect of 25 years of evidence-based medicine, the problem of bias is still prevalent in clinical research and its publication and in clinical practice. Its effect can lead to flawed research, misleading publications and ultimately patient harm. Aim: To draw attention to the commonest types of bias and how they influence clinical research, thinking and practice. Methods: This is not a systematic review but draws on the authors’ personal experience as clinical researchers, teachers, systematic reviewers and as arbiters of conflicts of interest for Cochrane. We describe the ones most relevant to oncology and give examples mainly from the literature on pulmonary metastasectomy. Results: There are two broad kinds of bias: technical bias, seen in the way research is conducted and published, and cognitive bias, the way in which beliefs, previous experience and thinking influence practice. The examples illustrate how common and diverse they are. Conclusion: These biases are widespread and influential and may actually cause harm. We are all susceptible to them and need to recognise them in ourselves and others and in what we read.