Assessment and evaluation are not optional for school librarians who want to be valued as full-fledged instructional partners. School librarians must document evidence of practice. Classroom teachers plan, implement, and assess student learning outcomes and then evaluate the effectiveness of their instruction. If school librarians are to be considered and serve as equal partners with classroom teachers and specialist colleagues, then comprehensive collaboration will include co-planning, co-implementation, co-assessment, and co-evaluation. Sharing the responsibility for assessment is a way for school librarians to be valued members on their schools' academic teams. Developing assessment expertise through coteaching is a pathway to leadership for school librarians.