Most articles on energy and economics focus on aspects such as the price of petroleum, the cost of electric power, or the demand for domestic air conditioning or industrial heat. Those topics are about the factors driving producers to invest in different sources of supply, and the factors affecting demand. Instead, this article is about how energy acts as a crucial factor of production and as a driver of, or barrier to, economic growth. It also explains how it happens that conventional economic theory neglects the role of energy, and discusses the implications of that neglect in terms of explaining past economic developments and forecasting the future. This article presents the key thermodynamic-based concepts for studying the relationships between energy and the economy, and finds that upcoming energetic constraints mean the neo-classical illusion of a global, perpetual economic growth machine will fail, and most likely sometime soon.