The concept of ecological limits - or planetary boundaries, in contemporary terminology - is sometimes regarded as an unwelcome constraint on politics, a kind of natural determinism that limits human autonomy or justifies reactionary values. This suspicion of natural limits occurs on both right and left, whether in the form of libertarian techno-utopianism or certain strands of eco-socialism. This paper argues in favor of a recognition and discourse of limits. Drawing on the civic republican tradition and its focus on non-domination, common goods, and the vulnerability of the polity to external forces, the paper makes three main arguments. First, attention to natural constraints and limits acts as an important check on authoritarian political and economic ambitions, which tend to rely on notions of the infinite malleability and resiliency of humanity and nature. Second, human relationships with the natural world yield a variety of common social and political goods. Such goods are threatened when ecological limits are breached. Third, the defense of such goods creates an important basis for individual and collective resistance to political and economic forces that might exercise domination through the state, technology, or the market. In making these arguments, the author draws especially on the work of John Barry, Iseult Honohan, and Philip Pettit.