This paper aims at introducing the developments of learning studies in Hong Kong and examining its impacts on teacher professional development. A learning study combines a “design-experiment” (Brown, 1992; Collins, 1992) with the “lesson study” model from Japan (e.g. Stigler & Hiebert, 1999) and the “teaching research group” in China (Ma, 1999). Before the invention of the learning study, the variation theory (cf. Marton & Booth, 1997) was used to analyse classroom teaching. The theory provided a useful framework to examine the inter-relationship between how teachers teach the same concept and what students have learned. Thereafter, the theory was used to support instructional design, with active participation of researchers and teachers. The notion of learning study then emerged and flourished in Hong Kong. Many schools have implemented learning studies to improve teaching and learning in one way or another. Some learning studies are in a more ‘design-experiment’ type with the focus on the theoretical development of the variation theory whilst other learning studies are more “lesson-study” type with emphasis on school-based development. Subsequently, the impacts of these two types of learning studies on teacher professional development are found to be quite different.