This article discusses the concept of planetary boundaries that has been advanced by a group of leading ex- perts around Johan Rockström. I place the concept of planetary boundaries in the larger framework of the emerging research paradigm of earth system governance, welcoming it as a crucial contribution that defines the overall goals of governance. Yet I also elaborate on the political conflicts that surround the identification of planetary boundaries, which are, in the end, a social construct. I then explore the policy and governance responses that may follow from the planetary boundary approach. In the conclusion, I point to several research challenges that flow from the current state of knowledge on planetary boundaries.