The evidence is overwhelming that ghosts, supposedly disembodied spirits of the dead, are not real. The notion of ghosts derives from ancient tendencies towards animism, the practice of anthropomorphizing non-human objects. It was also likely formulated as an effort to come to terms with death, and to commune with one's ancestors. Once ghost legends were initially imagined up, confirmation bias - the inclination to see what we want to see - took over, and perpetuated the phenomenon in human culture.
When considered without wishful thinking, the idea of ghosts is filled with logical holes. For instance, why are reports of ghosts less numerous in cities that have been inhabited for many thousands of years, such as many in the Near East, than in those established relatively recently, such as less than 150 years ago? We would expect the opposite to be the case. Why, despite the existence of millions of video cameras for at least a couple decades, has no one ever captured a convincing video of poltergeist activity? Why is all the evidence for ghosts purely anecdotal?
Given an observed tendency to believe in ghosts even in the absence of any personal experiences whatsoever, and the lack of any videographic evidence that can be authenticated, it seems extremely plausible that ghosts are not real. Scientists have proposed various explanations for why people believe in ghosts. Among them are sleep paralysis, the feeling you get when you wake up in the middle of the night, can't move your body, and feel a nearby presence; infrasound, which is too low to be audible but can still be felt subtly with the body and can cause a feeling of dread; and the existence of "orbs," ghost-like videographic relics common to all forms of artificial image capture.
Because believing in ghosts is a fun thing to do, many will continue to believe for quite some time, despite the shakiness of the evidence. A 2005 Gallup poll found that 32% of American adults believe in ghosts. However, the exponentiation of cheap video cameras seen in recent years, coupled with image analysis software, will keep piling the burden of proof on believers in ghosts, until eventually, all credibility in the notion of ghosts will evaporate like a specter in the fog.