As the school librarian at Aliamanu Elementary School, Patty (Patricia Louis) has focused on assessment with her teachers and students as a means to improve teaching and learning. Over the years, Patty and Vi (Violet Harada) have formed a collegial mentoring relationship, and this article provides a glimpse of what both discovered about building assessment into teaching practices through Patty's experiences. Patty and Vi are discovering that although self-assessment is not an easily mastered skill, it is a learnable one. It requires cooperation among all school professionals working with young learners. Librarians are strategically positioned to reinforce students' assessment skills. They not only support assessment work that is ongoing in classrooms, but they can also take the lead in introducing self-assessment strategies during library instruction. (Contains 6 figures.)