This article addresses the conceptual question "what is lesson study?" as an issue that arises in the context of the globalisation of lesson study as a method for improving teaching and learning beyond its presumed origins in the Japanese education system. To what extent can adaptations of the method in different national settings be interpreted as faithful representations of its practically significant "critical features" in the country of origin? In order to address this question, the article begins by examining the comparative classroom research by Stigler and Hiebert that culminated in the publication of "The Teaching Gap." This work is generally acknowledged to have been seminal for the global development of lesson study as a method for improving teaching and learning. Sponsored by the 1997 TIMMS testing programme, the research sought to explain pronounced differences in measured educational attainment between students of all ages in Japan, and the US and Germany. In the process Stigler and Hiebert discovered the extensive use of lesson study in Japanese primary schools as a school-based research method for securing consistency between learning goals and teaching methods. In doing so, they identified six principles which underpinned the method and pinpointed its practical significance. In this article, the author claims that the principles identified by Stigler and Hiebert can be used as a framework for assessing adaptations of lesson study in the context of globalisation, and connecting it to related methodological ideas that are internationally circulating. In particular, the author stresses links between lesson study, the tradition of classroom action research forged by Lawrence Stenhouse and his colleagues at the University of East Anglia, UK and the theory of variation developed in Sweden and Hong Kong by Ference Marton, Lo Mun Ling and others. Such links can deepen a theoretical understanding of "lesson study" and safe-guard it against a "cherry-picking" approach to its implementation in a context of globalisation. The article particularly highlights the importance of understanding the ways in which the organisational cultures of schooling in many countries shape and distort the implementation of lesson study. It argues for the greater involvement of school leaders and administrators in a form of "second-order action research" aimed at transforming the organisational context of teachers' work in classrooms and creating more space for them to spend less time as "test data managers" and more time as "lesson researchers" in accordance with the six principles outlined.