Déjà vu is a fleeting and subjective memory phenomenon where one has the feeling of familiarity at the same time as being aware this feeling is false. The contemporary understanding of déjà vu centers on a “conflict” between memory evaluations—recognizing something as familiar but knowing it should not be. Déjà vu is thus indicative of healthy memory functioning: it operates to raise to consciousness mismatches in memory processing. The various methodological approaches and theoretical explanations are reviewed in this chapter. The overarching principle is that it should be possible to incorporate explanations of this unusual and infrequent phenomenon into current theories of the functioning of memory.