This chapter presents a critical analysis of the concept of scaffolding as it has evolved over time. The analysis differs from existing reviews (Stone, 1998a; van de Pol, Volman & Beishuizen, 2010) in at least two ways. First, rather than assuming that scaffolding is a metaphor that needs to be formalized with a normative framework, we closely analyze source texts to uncover the epistemology of scaffolding. Second, rather than exclusively focusing on the origins of scaffolding, we use later work from the original line of research to augment and clarify its origins. The analysis begins by examining the current controversies surrounding the common notion of scaffolding as a metaphor for support. After reviewing the history of scaffolding and its theoretical foundations, we explore the structure and function of scaffolding. We examine the implementation of scaffolding in human tutoring and intelligent tutoring systems. Our analysis suggests that key components of expertise have a larger role to play in scaffolding than appears in common practice. In particular, we argue that the most effective scaffolding makes expertise visible. [Published in: R. Sottilare, A. Graesser, X. Hu, & B. Goldberg (Eds.), "Design Recommendations for Intelligent Tutoring Systems. Vol. 2 of Adaptive Tutoring," (pp. 327-340). Orlando: U.S. Army Research Laboratory.]