Full Stage Lay Summary Monitors that use the electroencephalogram to assess how deeply anaesthetised patients are when undergoing surgery are now widely available. The optimal depth at which anaesthetics should be given is, however, unknown. Recent observational studies have shown a 20% increase in mortality in patients undergoing major surgery who receive relatively deep anaesthesia. We plan to perform a large scale randomized trial of deep versus light anaesthesia to definitively answer the question of whether anaesthetic depth alters surgical outcome. In particular we will look at death rate at one year and also whether there are differences in other complications of surgery and anaesthesia, including wound infection, cardiovascular and neurological complications, pain and awareness. There are over 450,000 general anaesthetics administered every year in New Zealand. This study will have important implications for how we should run our anaesthetics and the findings will be easily translated into daily practice.