Japanese lesson study (LS), jugyu kenkyuu, is becoming an increasingly popular and important model for teacher learning and professional development (PD) worldwide. The spread of lesson study globally as well as in Singapore schools has been phenomenal. In Singapore, it was first introduced in 2004 and is now widely implemented as a tool for promoting professional learning communities (PLCs). This rapid diffusion of lesson study in many educational systems raises questions related to the quality of lesson studies practiced in schools as well as its sustainability, especially with change of school leadership. A national survey and case studies of schools implementing lesson study in Singapore have provided some insights into these issues. The hope is that lesson study will be practiced in the right spirit and substance and not be a passing fad in schools.