Accident investigation and risk assessment have for decades focused on the human factor, most notably in the form of 'human error'. Countless books and papers have been written about how to identify, classify, eliminate, prevent and compensate for 'human error'. This preoccupation with failure is near universal and can be found in all fields of application. One consequence of this has been a bias towards the study of performance failures, leading to a neglect of normal or 'error-free' performance. The common, unspoken assumption is that failures and successes have different origins and that there therefore is little to be gained from studying the latter. Erik Hollnagel argues strongly that this assumption is incorrect and that it is impossible to attain safety by eliminating risks and failures. Instead it is better to study why things go right, and to find ways to support and amplify that. The aim of this book is to present a single, simple but powerful principle for human performance that can be used to understand both positive and negative outcomes. The ETTO Principle reflects the common trait that people in their work naturally adjust what they do to match the conditions - to what has happened, to what happens, and to what may happen. It proposes that it is normal for people in work situations to adjust their performance by means of an efficiency-thoroughness trade-off (ETTO) - usually by sacrificing thoroughness for efficiency. The trade-off can be due to a lack of time, lack of resources, work and company pressures, lack of information, etc. The ability of people mutually to adjust their performance is the reason why things go right. Yet in some cases the adjustments may combine in an unforeseen way and lead to adverse outcomes. These outcomes are nevertheless due to the very same processes that produce successes, rather than to errors and malfunctions. The ETTO Principle obviates the need for specialised theories and models of failure and 'human error' and offers instead a viable basis for more effective and just approaches to both reactive and proactive safety management.