The term ”déjà vu” was first introduced around the 1890s in order to separate thephenomenon from other paramnesias, but a clear consensus on its definition was not reacheduntil mid 20th century. Since the middle of the 19th century, several dozens ofparapsychological, pseudoscientific and scientific theories have been proposed to explain thedéjà vu phenomenon, ranging from “messages from God” to “delayed neural transmissionspeed”. Most scientific theories can be divided into four categories: dual-processing,neurological, memory and attentional. This paper discusses and compares some of thesetheories. Memory and attentional theories are concluded to have most explanatory power andpotential to demystify the phenomenon through future research.