A tiger in the arctic. Sunflowers in a coral reef. Would they thrive -- or even survive -- in those ecosystems? Probably not, despite being strong and resilient species. Similarly, even well-designed, evidence-based professional learning must occur in a supportive, rather than endangering, ecosystem to take root and thrive. Ecosystems that support professional learning have multiple complex components and function in a dynamic manner. For professional learning to stick, leaders must attend to those components, shifting attention away from designing and implementing professional learning in isolation and toward sustaining the entire ecosystem. This article begins by describing the components and dynamics of the professional learning ecosystem. It goes on to highlight key roles leaders play in ensuring a thriving professional learning ecosystem, not only in allocating time and space but also in the messages they send and the practices they model.