This paper is about a design-based research project which evaluated the novel use of Lesson Study for assessment (LSfA) purposes. It starts by explaining the principles and design of LSfA procedures based on a Lesson Study model and dynamic assessment principles. It outlines the training and support provided to six Lesson Study teams in three primary and three secondary schools, each involving two class teachers and the school's SEN coordinator. The evaluation findings focused on the LSfA processes and procedures, for example, the outcomes of LSfA for teacher confidence and knowledge, assessment capabilities and continued benefits. The LSfA process was seen to involve processes such as honest and constructive observations, analysing lessons to see what to change and using knowledge about learning difficulties. Some of the context factors that supported and inhibited the process are also outlined. Analysis of the pre-post LSfA pupil assessments showed a decrease in identifying areas of pupil difficulties and an increased identification of enabling factors. This is interpreted as evidence for the LSfA having dynamic assessment potential. The paper concludes with an account of how the LSfA procedures were adapted in response to this evaluation and suggestions for future development and research.