It is essential that we find out what opportunities exist for the improvement of energy efficiency, and their limitations, since this is the most cost-effective way to reduce greenhouse gas emissions. This study presents a method for identifying potential technologies which can contribute to the long-term improvement of energy efficiency. An objective measurement of maximum improvement is derived by calculating the theoretical minimum specific energy required to perform a task. Next, a list is drawn up of technologies that could conceivably improve efficiency, extending beyond those normally found in literature on the subject. This method is applied to three major energy-consuming branches of industry: paper and board; iron and steel; and nitrogen fertilizer. Each of the studies provides an in-depth analysis of the industry and an extensive survey of possible ways to improve it.