Although increasing emphasis is being placed on mathematical justification in elementary school classrooms, many teachers find it challenging to engage their students in such activities. In part, this may be because the teachers themselves have not had an opportunity to learn what it means to justify solutions or prove elementary school concepts in mathematics. The authors used a lesson on fraction division to explore how elementary school teachers can be supported to develop an understanding of mathematical justification. Taught by an instructor in an integrated mathematics content and methods course for prospective teachers, the lesson topic--fraction division--was chosen because of prospective and practicing U.S. teachers' well-documented struggles with it. The authors' analysis of the lesson illustrates the instructor's attempt to help future teachers develop four elements of knowledge that are essential for teaching mathematics with a focus on justification: (1) Knowing what counts as valid justification for a given answer; (2) Familiarizing oneself with the struggles elementary school students may have; (3) Understanding how mathematical topics connect across operations and number systems; and (4) Knowing how to teach in a way that supports mathematical reasoning. (Contains 2 figures.)