Wastewater treatment is an energy and resource intensive process. The treatment objective is to produce an effluent that meets environmental discharge limits. As the limits become ever more stringent, it is predicted that energy consumption will increase. This research focuses on the development of an energy auditing methodology for wastewater treatment plants (WWTPs) and investigates potential measures for increasing plant energy efficiency. In-depth energy audits and water quality testing were performed simultaneously on four WWTPs in Ireland. Two small plants (400 – 500 P.E.) and two medium sized plants (12,000 P.E.) were chosen for this study. These plants are representative of many WWTPs across Ireland, with 87% of all Irish WWTPs being smaller than 10,000 P.E. Additionally, one large plant (50,000 P.E.) was selected for a preliminary evaluation of energy consumption and distribution. The plant energy audits identified numerous opportunities to improve energy efficiency. Plant layout issues, ineffective control/automation systems and various electrical inefficiencies were discovered. Another important finding of this study showed that influent composition can have a large effect on the interpretation of the energy efficiency results and perceived plant performance. Energy audits alone do not tell the full story and parallel water quality monitoring is required in order to make comparisons from plant to plant.