When studying human brains in relation with digital technologies, or digital brains, a relatively recent technology may prove particularly promising to do so: Brain-Computer Interfaces (BCI). Indeed, BCI can decode measures of users' brain activity in real-time, in order to enable direct control of computers via brain activity or to monitor users' mental states when interacting with technologies (socalled neuroergonomics). This chapter presents an introductory overview of this technology, i.e., it describes its motivations, brief history, components, principles of operation and various applications, e.g. for assistive technologies, neurorehabilitation or safety, performance and user experience assessment and optimisation. It also touches on the various current limitations of this technology, which makes it rather different from the science-fiction-like representations it may evoke. Altogether, we hope this chapter can offer a brief but clear glimpse into what BCI can and cannot do, and motivate readers to possibly consider them in their future research and/or developments.