Recent years have witnessed an increased emphasis on the importance of evidence in behavioral healthcare practice (APA, 2006; Bieschke, Fouad, Collins, & Halonen, 2004; Fisher & O’Donohue, 2009) . However, the integration of science and clinical practice has not gone smoothly (e .g ., Boisvert & Faust, 2006; Norcross, Klonsky, & Tropiano, 2008) . This chapter addresses the integration of scientific evidence into practice, including the importance of basing practice on scientific evidence, differing perspectives on the definition and quantification of “evidence,” reasons for the apparent gap between science and practice, and suggestions for bridging this gap and improving mental healthcare for the twenty-first century . Although parts of the chapter focus on issues within the field of psychology, all issues discussed are highly relevant to behavioral healthcare broadly defined .