A state of ignorance about a proposition can result from two distinct sources: uncertainty about what the correct answer actually is, and unawareness of what the answer might possibly be. Uncertainty con-cerns the strength of one's evidence, whereas unawareness concerns the conceptual components needed to articulate a proposition. This chapter discusses the implications of the distinction between uncer-tainty and unawareness for metacognition, and more specifically for the problem of what it takes to know that one knows and to know that one does not know. In particular, we relate the distinction between uncertainty-based unknowns and unawareness-based unknowns to the two-stage model proposed by Glucksberg and McCloskey for decisions about ignorance.