Online learning is growing rapidly across the United States within all levels of education, as more and more students and educators become familiar with the benefits of learning unconstrained by time and place. Across most states and all grade levels, students are finding increased opportunity, flexibility, and convenience through online learning. Teachers are discovering a new way to reach students, many of whom were not successful in traditional schools and courses. Administrators are exploring ways to offer a wider range of courses to students and professional development opportunities to teachers. This report provides a comprehensive overview of online learning by examining the basics--teaching and learning, evaluating academic success, professional development, technology and other topics. Two appendices are included: (1) Definitions; and (2) Additional resources. (Contains 34 footnotes.)