Data centres are at the heart of our modern-day digital infrastructure. Our use of and reliance on data is only set to grow and therefore data centres will also have to grow in number and size with improved reliability, resilience and efficiency. Key to this is the power systems of data centres. These systems, from the simplest to those that enable concurrent maintainability, where the service can be continued despite maintenance operations, and Fault Tolerance, where the facility can withstand any primary fault and survive, have become main features of data centre design. The aim of this document is to provide comprehensive guidance to anyone involved in the specification, design, commissioning, operation and maintenance of data centre power systems. It will be relevant to manufacturers of components (such as transformers, emergency generators and uninterruptible power supplies (UPSs)), specifiers of components, operators of data centres (including facility management organisations) and training organisations (including colleges and universities).