Motivation is a very complex topic. Various fields of study have defined motivation differently. Education is one of those fields. Motivation is one of the important topics in education that is essential for improvement of students` achievement. Motivation in education depends on different variables such as age, culture, gender, learning environments and so on. This study is concerned with one of the learning environments namely online learning. The primary task of instructors in online learning is to motivate their students. This paper offers literature review on motivation of adult learners enrolled in online courses. This paper also provides insights on the essential theories and perspectives of motivation as well as brief information about ARCS model developed by Keller. The study poses some important questions that intend to challenge teaching skills of its readers.