The use of new therapeutic and diagnostic technologies has become commonplace in modern medical practice. To avoid both clinical disappointment and the waste of money, health, and lives, the introduction of these technologies will have to be based on evidence that these technologies will do more good than harm. The evidence supporting their use should be derived using research methods designed to deal with placebo effects, confounders, and biases. Some of these methods, and the rationale for their use, are discussed in this article. Although the value of evidence derived from randomized controlled trials is stressed, the importance of reviewing critically the methodological details of such trials and interpreting their results with caution is emphasized. The benefits and risks of relying on case-control and cohort studies are reviewed.