Passive brain–computer interfaces (passive BCI; pBCI) have been introduced and formally defined almost a decade ago and have gained considerable attention since then. In this chapter, we clarify some points of confusion and provide a perspective on the past, present, and future of the field of passive BCI. This perspective concerns a key aspect with regard to which various pBCI-based systems differ from each other: interactivity. The more interactive a system is, the more responsive it is, the more autonomous, and the better capable of adaptation. Along these lines, we identify and describe four relevant categories of systems with varying levels of interactivity: mental state assessment, open-loop adaptation, closed- loop adaptation, and automated adaptation. We give examples of past and current research for each of these categories. The latter three are collectively introduced as neuroadaptive systems. This perspective and formal categorisation helps to highlight human–computer interaction aspects that are relevant for the design of pBCI-based systems and points to possibilities for future research and development into passive BCI, implicit interaction, and neuroadaptive technology