This study reviewed dropout research in online learning, particularly focusing on nontraditional students and examined what progress has been made since Garrison's 1987 review. This study categorized factors identified from literature into student characteristics, student prior skills, external factors, and internal factors, all of which are based on Rovai's persistence model (2003). Current status of dropout studies for adult learners in distance learning is discussed and future research directions are proposed. (Contains 1 figure and 1 table.)