The work addresses brain-computer interface (BCI) technology, its origins in the history of neuroprostheses, and the current shift from medical to commercial applications. The insights gained are applied to the notion of the interface and its significance in the context of BCI. This gives rise to new challenges, but also opportunities for design. A comparison is made between the human brain and the computer, so as to better define the individual components of the acronym BCI. This is followed by an explanation of existing theories, such as the Turning Theory, Searle’s Chinese Room, or the Theory of Action. The interview with Nelson Steinmetz, the head of design at NextMind, provides a greater understanding of the task of interfaces, which is to mediate between the brain and computer.