Complexity issues in the ecological aspects of chemical engineering are of two kinds: the mutual interactions of many multi-scale systems and the lack of consilience in the objectives of di6erent disciplines that consider the economic, philosophical, cultural, and scienti7c and engineering aspects, respectively. This paper discusses the second kind of complexity and the ecological issues in the di6erent disciplines. Economic value, as expressed by market price, depends on whether limits on resources and the Earth's services due to the scale of human technological activities are taken into account. Human activities relative to the Earth's ecology require sustainability consideration. Two important stands on sustainability are discussed. Mainstream sustainability is a stand for continued economic growth to foster advances in technology to overcome the limits of the Earth. It requires technological activities to follow green chemistry and green engineering principles to develop innovations that are ecologically considerate. Environmental sustainability is a stand to stay within forecasted limits in the resources and renewability capacity of the Earth. The technology for this further includes using the principles of industrial ecology to locate ways to use wastes and recycle products as source materials for other processes to make and close material cycles. Chemical engineering innovations for increased atomic utilization of reactants, e=ciency in energy use, dematerialization, non-toxicity, recyclability, and creative systemic cycling of materials for waste management are good for the ecology. Case studies are given showing the utility of following the principles developed for good ecological practice.