Dual enrollment--the arrangements by which high school students take college courses during the junior and senior year--is a promising "next best thing" for states wishing to increase the number of underrepresented students gaining a postsecondary credential. Dual enrollment also has the potential to save money for families and taxpayers and shorten time to degree. To make dual enrollment a centerpiece of a strategy to improve college access and success, however, requires shifts in typical dual enrollment policy and legislation and a new way of thinking about its mission. By "adding" supports at the front end--in eleventh and twelfth grades--in order to enable young people to succeed in college-level courses in high school, states can potentially "subtract" from the total expense of educating a young person. "Add and Subtract" is a policy primer for states wishing to implement dual enrollment as a strategy for increasing college credentialing rates of underrepresented students. It provides: (1) An overview of dual enrollment and a rationale for its expansion; (2) Guidelines (including funding models) for states wishing to implement dual enrollment for a wider range of students; and (3) Brief case studies of substantial dual enrollment programs that serve a wide range of students--Florida and Utah and College Now at the City University of New York--and offer lessons for an expanded mission for dual enrollment. (Contains 4 tables and 28 endnotes.) ["Add and Subtract" is part of Double the Numbers, a Jobs for the Future Initiative.]