An extensive literature review and a study of unpublished documents were undertaken to summarize information on the performance, design and operation of "northern" extended aeration waste water treatment plants. This information was compared with that for 'non-northern' plants and National Sanitation Foundation (NSF) plants, the latter providing an indication of optimum performance.The study revealed that the northern plants generally performed more poorly than their non-northern counterparts and much worse than the NSF plants.Reasons for poor performance of the northern plants were related to design and operation deficiencies. The most frequently observed conditions indicating design errors were organic and hydraulic overloading. The most common operating errors were failure to scrape hopper-type clarifiers and failure to maintain sludge returns operative.Very few design and operating problems relating specifically to northern plants were identified since many of the smaller plants were encapsulated for freeze protection.