The study of children's prospective memory has gained new momentum over the past 20 years and is now an active area of research in cognitive development. Yet, this resurgence has been accompanied by significant challenges that offer important lessons and insights for other areas of developmental science. In this article, I provide an overview and theoretical accounts of the development of children's prospective memory. I then describe three puzzling findings that remain unresolved in children's development of this kind of memory: (1) the contradictory effects of delay length on prospective memory performance, (2) why reminders sometimes fail to improve prospective memory performance, and (3) why parent reports and behavioral measures of prospective memory might fail to correlate. I discuss how each challenge can be applied broadly to other areas of children's development. I conclude by highlighting challenges and opportunities as the field moves forward.