The American Association of School Librarians' (AASL) "Standards for the 21st-Century Learner" (2007), with their emphasis on multiple literacies and the social context in which information skills develop, embrace instruction that engages students' critical thinking about the changing communications environment. An inquiry-based approach to learning is advocated, one that places the use of questions in a prominent role in the teaching process. While questions of all types have a useful pedagogical function, essential questions are particularly well suited to the task. Essential questions focus on the development of students' ability to sustain inquiry and critical thinking, which is also at the heart of the skills, dispositions, responsibilities, and self-assessment strategies of the AASL standards. A growing body of literature about essential questions provides a rich resource for librarians, teachers, and educational administrators. This article discusses some of the most significant and unique attributes of these questions as focal points for teaching.