This chapter reviews the techniques currently applied to study brain function during sleep deprivation (SD) as opposed to the consequence of SD. It provides a bird's eye view of functional imaging studies performed on healthy young adult volunteers to date and comment on how this research has evolved the conceptualization of how SD modulates behavior. The first functional imaging studies involving SD utilized positron emission tomography (PET). Based on the initial findings, cognitive domain and task difficulty was proposed as determinants of the neural response to SD. It was postulated that changes in dopamine signaling in the SD state contributed to the change in functional connectivity, an idea reprised when discussing risky decision making in SD. The interaction of SD and circadian effects, including the effects of chronotype, could be a further target of functional neuroimaging studies, including the effect of countermeasures such as naps and stimulants.