In this paper, we analyze 2 lessons by 1 teacher that took place during a lesson study (LS) cycle for a team of elementary mathematics teachers. We analyze the data on 2 levels: macro-level analysis, dealing with lesson structure and setting; and micro-level analysis, zooming in on mathematical tasks and the quality of whole-class discussion. We analyze teacher participation in LS to identify the main mechanisms leading to changes in teaching observed between the 2 lessons. We argue that collaborative noticing, collaborative awareness, and brainstorming are the core mechanisms of teacher change in the course of LS.