This paper argues on both theoretical and empirical grounds that, beyond a certain point, there is an unavoidable conflict between economic development (generally taken to mean 'material economic growth') and environmental protection. Think for a moment of natural forests, grasslands, marine estuaries, salt marshes, and coral reefs; and of arable soils, aquifers, mineral deposits, petroleum, and coal. These are all forms of 'natural capital' that represent highly-ordered self-producing ecosystems or rich accumulations of energy/matter with high use potential (low entropy). Now contemplate despoiled landscapes, eroding farmlands, depleted fisheries, anthropogenic greenhouse gases, acid rain, poisonous mine tailings and toxic synthetic compounds. These all represent disordered systems or degraded forms of energy and matter with little use potential (high entropy). The main thing connecting these two states is human economic activity. Ecological economics interprets the environment-economy relationship in terms of the second law of thermodynamics. The second law sees economic activity as a dissipative process. From this perspective, the production of economic goods and services invariably requires the consumption of available energy and matter. To grow and develop, the economy necessarily 'feeds' on sources of high-quality energy/matter first produced by nature. This tends to disorder and homogenize the ecosphere, The ascendance of humankind has consistently been accompanied by an accelerating rate of ecological degradation, particularly biodiversity loss, the simplification of natural systems and pollution. In short, contemporary political rhetoric to the contrary, the prevailing growth-oriented global development paradigm is fundamentally incompatible with long-term ecological and social sustainability. Unsustainability is not a technical nor economic problem as usually conceived, but rather a state of systemic incompatibility between a economy that is a fully-contained, growing, dependent sub-system of a non-growing ecosphere. Potential solutions fly in the face of contemporary development trends and cultural values.