The most important challenge that we presently face is reconciling the needs of a growing human population with our ability to mobilize the resources of the planet in order to support those needs in a sustainable manner. For human societies to function, it is necessary that they ‘consume’ energy and materials. Although a widely used term, it is incorrect to say that either materials or energy are consumed, as shown in Chapter 2. What is really being consumed is exergy. However, there are important differences in this respect between fuels and materials. Once a fuel has been burnt, it is not possible to recover the dispersed energy quanta, whereas with materials it is theoretically possible to recover any dispersed material, albeit with an energy expenditure. This chapter examines why it is that we are using ever-increasing amounts of resources. So far, we do not appear to be seeing any indications of real resource scarcity, but is this going to continue to be the case forever? Meanwhile, the consequences of consumption are the production of low grade heat, wastes and pollutants. We are now seeing signs of our activities on a global scale; is this where the limits lie?