The objective of this paper is to review and synthesize two interrelated topics in the adult metacomprehension literature: the bases of metacomprehension judgment and the constraints on metacomprehension accuracy. Our review shows that adult readers base their metacomprehension judgments on different types of information, including experiences with current tasks and pre-formed expectations of performance affected by factors such as self-perception of ability. We propose a model that shows the anchoring and adjustment mechanism (Tversky and Kahneman, "Science" 185:1124-1130, 1974) underlies metacomprehension judgments. Specifically, due to test uncertainty, people may judge future comprehension performance by starting with an anchor such as pre-formed performance expectations and then (insufficiently) adjust away from it based on experiences with current tasks. This anchoring and adjustment model of metacomprehension judgment sheds light on what constrains metacomprehension accuracy. We conclude that two main accuracy constraints are the anchoring effect and the poor diagnostic validity of experiential cues. Based on the review, we discuss avenues for future research that will further our understanding of the mechanisms underlying metacomprehension.