This document specifies the brain–computer interface (BCI) reference architecture (RA). The BCI RA defines the key characteristics of BCI systems, provides a common language for stakeholders, and addresses their concerns. It also provides guidance to system architects on efficient BCI system design, typical implementation categories and effects of using BCI technology in developing applications. Additionally, this document serves as a technical reference for architects, manufacturers, vendors, service providers, regulators and the public to better understand the BCI architecture, its functions and the underlying technology that supports the BCI systems.