Corporations interact with society and the physical and biological environment in ways that affect both sides. In this capacity, corporations play an important role in the sustainability of a region or country. Symmetrically, a corporation's sustainability depends on the sustainability of its wider environment. We present here a multistage fuzzy reasoning model which can be used to assess a corporation's sustainability. The model has two fundamental components: human and ecological. The human component has four inputs: economic, political, knowledge, and welfare; the ecological also has four inputs: air, water, land, and biodiversity. Each of these eight components has a number of basic inputs suitable to a given corporation, such as: renewable energy use, fossil fuel use, solid waste generation, wastewater emissions, employee benefits, injury rates, total revenue, research and development expenditure, etc. This model, which is based on the Sustainability Assessment by Fuzzy Evaluation (SAFE) model [Phillis and Andriantiatsaholiniaina, “Sustainability: An Ill-Defined Concept and its Assessment Using Fuzzy Logic,” Ecol. Econ., vol. 37, p. 435-456 (2001)], can be used to assess a corporation's sustainability and record its historical evolution. Equally importantly, sensitivity analysis of the model reveals the most important basic indicators which affect corporate sustainability to which decision makers should place special attention.