Two studies were conducted to investigate effects of domain knowledge on metacognitive monitoring across the life span in materials of different complexity. Participants from 4 age groups (3rd-grade children, adolescents, younger and older adults) were compared using an expert-novice paradigm. In Study 1, soccer experts' and novices' ease-of-learning judgments (EOLs), judgments of learning (JOLs), and confidence judgments (CJs) were contrasted when memorizing soccer-related word pairs. In Study 2, monitoring judgments (i.e., a rating of global comprehension, JOLs, and CJs) were collected in regards to a soccer-related narrative. The results of both approaches showed that experts' better memory performance obtained in both studies was not always accompanied by advantages in monitoring performance. In Study 1, experts of all ages outperformed novices in monitoring accuracy. In Study 2, no benefits of expertise on monitoring were found; in children, novices even surpassed experts in monitoring quality. In both studies, the most consistent influence of previous domain knowledge on monitoring performance concerned more optimistic judgments of experts compared with novices, regardless of stimuli and recall format. In sum, our results document a twofold effect of expertise on monitoring. Although domain-specific knowledge enhances monitoring performance in some situations, more optimistic estimates, presumably due to the application of a familiarity heuristic, typically reduce experts' monitoring accuracy.