"Dual enrollment" programs allow high school students to take college courses and earn college credits while still attending high school. Such programs, also referred to as "dual credit" or early college programs, are designed to boost college access and degree attainment, especially for students typically underrepresented in higher education. "Dual enrollment programs" support college credit accumulation and degree attainment via at least three mechanisms. First, allowing high school students to experience college-level courses helps them prepare for the social and academic requirements of college while having the additional supports available to high school students; this may reduce the need for developmental coursework. Second, students who accumulate college credits early and consistently are more likely to attain a college degree. Third, many "dual enrollment" programs offer discounted or free tuition, which reduces the overall cost of college and may increase the number of low socioeconomic status students who can attend and complete college. This What Works Clearinghouse (WWC) intervention report presents findings from a systematic review of "dual enrollment" programs conducted using the WWC Procedures and Standards Handbook, version 3.0, and the Transition to College review protocol, version 3.2. The What Works Clearinghouse (WWC) identified five studies of "dual enrollment" programs that both fall within the scope of the Transition to College topic area and meet WWC group design standards. Two studies meet WWC group design standards without reservations, and three studies meet WWC group design standards with reservations. Together, these studies included 77,249 high school students across the United States. "Dual enrollment" programs were found to have positive effects on students' degree attainment (college), college access and enrollment, credit accumulation, completing high school, and general academic achievement (high school), with a medium to large extent of evidence. For the staying in high school, college readiness, and attendance (high school) domains, "dual enrollment" programs had potentially positive effects with a small extent of evidence. "Dual enrollment programs" were found to have no discernible effects on general academic achievement (college) with a small extent of evidence. The following are appended: (1) Research details for Berger et al. (2014); (2) Research details for Edmunds et al. (2015); (3) Research details for An (2013, EJ1009522); (4) Research details for Giani et al. (2014, EJ1026262); (5) Research details for Struhl and Vargas (2012, ED537253); (6) Outcome measures for each domain; (7) Findings included in the rating for the degree attainment (college) domain; (8) Findings included in the rating for the college access and enrollment domain; (9) Findings included in the rating for the credit accumulation domain; (10) Findings included in the rating for the completing high school domain; (11) Findings included in the rating for the general academic achievement (high school) domain; (12) Findings included in the rating for the staying in high school domain; (13) Findings included in the rating for the college readiness domain; (14) Findings included in the rating for the attendance (high school) domain; (15) Findings included in the rating for the general academic achievement (college) domain; (16) Description of supplemental findings for the degree attainment (college) domain; (17) Description of supplemental findings for the college access and enrollment domain; (18) Description of supplemental findings for the completing high school domain; (19) Description of supplemental findings for the staying in school domain; (20) Description of supplemental findings for the college readiness domain;and (21) Description of supplemental findings for the college readiness domain. Also provided are WWC Rating Criteria, and a Glossary of Terms.