The U.S. Department of Education provides a great deal of support to education each year through formula funding (e.g., Title I, Title III services to students) and discretionary grants (e.g., Title III pro-fessional development grants, Safe Schools/Healthy Students projects). However, because evaluations of these projects, and many educational programs, often are not as systematic as might be expected, it may be difficult to determine whether the monies spent have resulted in increased long-term academic achievement for students. The purpose of scientifically-based research within NCLB is to identify and disseminate conclusive information on "what works" in education, with an empha-sis on determining what instructional input (curricula, instructional techniques) works to increase student outcomes such as academic achievement and language proficiency. Scientific research, such as that used in medical research and other "hard" sciences, offers rigorous methodologies for studying what works, for determining whether a particular curriculum or instructional technique will result in improved academic performance (or increased language proficiency) for a majority of the children for whom it was designed. The purpose of this document is to provide a definition of "scientifically-based research", and the major elements necessary to conduct such research. These definitions, and those in the NCELA document "Definitions for No Child Left Behind: Assessment", also provide background for two others in the series: "Research and Evaluation that Works within NCLB Standards and Criteria for Evaluating Evidence-Based Research".