The lesson study approach is a systematic process for producing professional knowledge about teaching by teachers, and has spread rapidly and extensively in the United States. The learning study approach is essentially a kind of lesson study with an explicit learning theory--the variation theory of learning. In this paper, we argue that having an explicit learning theory adds value to lesson study, as the variation theory of learning serves as a source of guiding principles for the teachers when they engage in pedagogical design, lesson analysis and evaluation. Through the use of two Hong Kong learning study cases, one from each of the two major ways of conducting learning study, we demonstrate the power of variation theory in explaining and predicting the relationship between what has taken place in the classroom and what the learners learn, and subsequently identifying ways to improve student learning through promoting teacher professional learning in a learning study setting. (Contains 11 tables and 1 figure.)