Bias is a systematic error that distorts the true relationship between an event and its outcome. Bias will negatively affect the truth of the conclusions. In research, bias includes any systematic error in the design, conduct, or analysis of a study. Bias can occur at all stages of research, from the selection of the population, how treatment is provided, and how and when outcome measurements are made. In health care research, bias can result in a mistaken estimate of a treatment's effect or an exposure's effect on the course of the disease. This article stresses the importance of optimizing continued learning and patient care—how clinicians should understand and scrutinize the various biases that can exist in research reports.