Déjà vu, French for "already seen," is the sensation of feeling eerily familiar with a newly encountered object or situation. It's that slightly jarring state of mind that can be triggered by any number of stimuli: a photograph, a phrase, a hand gesture. The sense that you've been here before. Due to its ephemeral nature, there's very little scientific research on déjà vu. Although over 70% of people report having experienced the unsettling phenomenon, it does appear to affect people aged 15 through 25 more than any other age group. And according to How Stuff Works, it has been associated with temporal-lobe epilepsy. Some epileptics have reported feelings of deja vu that occur prior to and even during seizures. Theories abound as to the cause of déjà vu. Simple wish fulfillment? A cerebral cross-wiring that confuses the present with the past? A pre-existing anxiety acting out? The subject is obviously still ripe for scientific and artistic exploration.

haven't you asked this question already?

A temporary rift in the space-time continuum.

I get the feeling I've read this question before...Hmmmmmm

Check this out: Déjà vu (pronounced /ˈdeɪʒɑː ˈvuː/ (help·info); French /deÊ’a vy/ (help·info) "already seen"; also called paramnesia, from Greek παρα para, "near" + μνήμη mnÄ“mÄ“, "memory") is the experience of feeling sure that one has witnessed or experienced a new situation previously (an individual feels as though an event has already happened or has repeated itself). The term was coined by a French psychic researcher, Émile Boirac (1851–1917) in his book L'Avenir des sciences psychiques (The Future of Psychic Sciences), which expanded upon an essay he wrote while an undergraduate. The experience of déjà vu is usually accompanied by a compelling sense of familiarity, and also a sense of "eeriness", "strangeness", or "weirdness". The "previous" experience is most frequently attributed to a dream, although in some cases there is a firm sense that the experience "genuinely happened" in the past. The experience of déjà vu seems to be very common; in formal studies 70% of people report having experienced it at least once. References to the experience of déjà vu are also found in literature of the past, indicating it is not a new phenomenon. It has been extremely difficult to invoke the déjà vu experience in laboratory settings, therefore making it a subject of few empirical studies. Recently, researchers have found ways to recreate this sensation using hypnosis.[1]

I have read where deja vu is the brain recycling instant memories. Let's say you just picked up your wallet. Your brain usually stores that image and that memory away for later recall. It does nothing with it at the moment, but when you want that memory it presents it to you for review. Deja vu is the brain storing an image and memory and then instantly presenting it back to you as a previously stored memory. It both stores it and recalls it at the same time. Since it is stored in long term memory, you think it is something that happened a while back when it actually happened literal seconds ago.

I've just been told that its supposed to be when your soul travels forward in time and experiences something before your body does. When your body reaches that point in time you get deja vu. :)

A collage of similarities from previous incidents being recognized as such

The brain recognising something in the subconscious mind fractionally before the conscious mind

It's a glimpse into the workings of the third eye.

The information gets short-circuited into long term memory before it hits short term memory, tricking you into thinking you've seen/been there before.

Feedback memory or compelling sense of familiarity.

I think it has to do with scent, A familiar scent from another time conjures up the old memory and relates to the current circumstance

Deja Drugs or Deja Booze

PEOPLE PEOPLE LISTEN DEJA VU IS CASUED BY THE MEMORY OF SITUATIONS EXPERIENCED IN DREAMS. FUN FACT: ABOUT 63% OF PEOPLE BELIEVE THAT DREAMS CAN PREDICT THE FUTURE

well when my daughter got pg the second time and the father suspected it wasnt his, he named her Deja because it happened to him before. (as in Deja Vu) As it turned out, it wasnt his but his dads. Yea. The girls have a stable life because we have been raising them for years.

Deja vu is caused when you are inable to differentiate from what you are experiencing and a similar experience you have had before either in a dream on in real life. Many places do look similar to other places but you will not allways get deja vue. You will tend to get it when you recall being in the place with a certain feeling that tags the experience and for some reason or another you get a very similar feeling which causes you to suddenly recall your last experience. You will not always remember where it was or how you got there in your last expereince and it probably was not the same place but just one which looked very similar. The reason for this is you only remember the place in relation to how you felt at a specific time when you were there. Most people get deja vu when drowsy or in a slight transe.