When judging the benefits and harms of health care and predicting patient prognosis, clinicians, researchers, and others must consider many types of evidence. Medical research evidence is part of the required knowledge base,and practitioners of evidence-based medicine must attempt to integrate the best available clinical evidence from systematic research with health professionals' expertise and patients' rights to be informed about diagnostic and therapeutic options available to them. Judging what constitutes sound evidence can be difficult because of, among other things, the sheer quantity,diversity, and complexity of medical evidence available today; the various scientific methods that have been advanced for assembling, evaluating, and interpreting such information; and the guides for applying medical research evidence to individual patients' situations. Recommendations based on sound research can then be brought forward as either guidelines or standards, and criteria exist by which valid guidelines and standards can be developed and promulgated. Nonetheless, gaps and deficiencies exist in current guidelines and in the methods for finding and synthesizing evidence. Interpreting and judging medical research involves subjective, not solely explicit, processes. Thus, developments in evidence-based medicine are an aid, but not a panacea,for definitively establishing benefits and harms of medical care, and the contributions that medical research evidence can make in any clinical or legal situation must be understood in a context in which judgment and values,understanding of probability, and tolerance for uncertainty all play a role.