  Generative AI (GenAI) systems offer unprecedented opportunities for
transforming professional and personal work, yet present challenges around
prompting, evaluating and relying on outputs, and optimizing workflows. We
argue that metacognition$\unicode{x2013}$the psychological ability to monitor
and control one's thoughts and behavior$\unicode{x2013}$offers a valuable lens
to understand and design for these usability challenges. Drawing on research in
psychology and cognitive science, and recent GenAI user studies, we illustrate
how GenAI systems impose metacognitive demands on users, requiring a high
degree of metacognitive monitoring and control. We propose these demands could
be addressed by integrating metacognitive support strategies into GenAI
systems, and by designing GenAI systems to reduce their metacognitive demand by
targeting explainability and customizability. Metacognition offers a coherent
framework for understanding the usability challenges posed by GenAI, and
provides novel research and design directions to advance human-AI interaction.
