Recently, there has been a rapid proliferation of lesson study groups in the United States. Since deep knowledge about lesson study is rare in the U.S., it is likely that some of these groups have an incomplete understanding of this Japanese practice. Some may focus on structural aspects of the process of lesson study or may mimic its superficial features, while ignoring the underlying rationale for them. The purpose of this study is to clarify some of the guiding principles behind the process of lesson study by identifying three categories of challenges that U.S. practitioners of lesson study may encounter at different "developmental stages" in their learning about the process. Another purpose is to help lesson study practitioners focus on core principles and more coherently define the purpose of their lesson study work, so that they can move beyond its procedural aspects toward richer, more sustainable practice.