This chapter is divided into two sections: bias within single studies and bias between studies in systematic reviews. Systematic reviews aim at avoiding many of the biases contained in traditional narrative reviews, but nevertheless there is still potential for numerous biases even in elaborate systematic reviews. Randomised controlled trials (RCTs) are considered the potentially most unbiased form of clinical epidemiological study designs if conducted appropriately. There are currently two methods to assess study quality: composite scales and the component approach. There are several methods to detect publication bias, including simple graphical and more complex statistical methods. There is empirical evidence that studies with significant results are published or cited earlier and more frequently than those with non-significant or unfavourable results. Limited evidence exists for outcome reporting bias, duplicate publication bias, language bias and database bias.