A decade ago, now-seminal work showed that children are strikingly unskilled at simple tool innovation. Since then, a surge of research has replicated these findings across diverse cultures, which has stimulated evocative yet unanswered questions. Humans are celebrated among the animal kingdom for our proclivity to create and use tools and have the most complex and diverse technology on earth. Our capacity for tool use has altered our ecological environments irrevocably. How can we achieve so much, yet tool innovation be such a difficult and late-developing skill for children? In this article, I briefly summarize what we know about the development of tool innovation, then discuss five outstanding questions in the field. With a focus on different empirical and theoretical perspectives, I argue that addressing these questions is crucial for understanding fully the ontogeny of one of humans' most notable skills.