This paper explores the theoretical framework of threshold concepts and its potential for LIS education. Threshold concepts are key ideas, often troublesome and counterintuitive, that are critical to profound understanding of a domain. Once understood, they allow mastery of significant aspects of the domain, opening up new, previously inaccessible ways of thinking. The paper is developed in three parts. First, threshold concept theory is introduced and studies of its use in higher education are described, including emergent work related to LIS. Second, results of a recent study on learning experiences integral to learning to search are presented along with their implications for search expertise and search education, forming a case illustration of what threshold concept theory may contribute to this and other areas of LIS education. Third, the potential of threshold concept theory for LIS education is discussed. The paper concludes that threshold concept theory has much to offer LIS education, particularly for researching critical concepts and competencies, and considerations for a research agenda are put forth.