This paper examines research on learning styles as related to online learning for adult learners. There is much disagreement regarding the definition of learning style. This paper defines it as an individual's preferred way of learning. The focus is on the extent to which learning styles are able to predict student success (e.g., grades, attitudes). The paper discusses nine different instruments that were used in various studies. Curry's model, which uses the metaphor of the layers of an onion, is used to categorize the instruments by theme for the sake of comparison. Criticisms of learning style research include: the vagueness of the construct "learning style"; the fact that the instruments are self-assessments; mixed results from research that searched for a relationship with online learning; and the difficulty of comparing different studies when online learning can include many different methods and technologies. Finally, I reach a conclusion regarding the suitability of studying the relationship of learning styles and success in online courses.