We developed the definition of an XL Bully dog by convening a group of experts and other stakeholders to define the physical characteristics of the breed type. This included representatives from the police, local authorities, devolved administrations and the Animal Welfare Committee, which advises the Government on animal welfare issues. We reviewed existing breed standards provided by breed registries in the United States and the UK and tested the standard in practice. We have provided further guidance to support owners in applying this.