Refers, in general, to evidence that best attains certain quality criteria for internal and external validity. This also refers to a principle that a desire to base health care decisions and policies only on evidence generated from the best study designs for establishing internal and external validity should not preclude using the best evidence that is available from other study designs. That is, the “best evidence” may be the best available evidence that is relevant for the evidence questions of interest. This does not necessarily mean that the best available evidence should be designated as being of high quality.