The all seeing eye was addopted by the Freemasons as a symbol for the Great Architect. The Illuminati had infiltrated the Freemasons back in the 1600s i think it was. Now the Illuminati use it as a symbol. Along with the Pentagram, Pyramids, the owl, triangles etc.
Oh noez! They must have infiltrated Wiki...
"The assumption that the Illuminati had owned certain symbols which they used to prove their existence to insiders and clever symbologists, can be widely found in novels. The following symbols are supposed to belong to those used by the Illuminati.
the all-seeing eye
the all-seeing eye as pyramidion (Great Seal of the United States)
the number 23
None of these symbols are historically related to the Illuminati. It is also highly implausible that an internationally operating conspiracy, as which the Illuminati are often labeled, would leave their symbols everywhere. The Illuminati only used one symbol for their "Secret School of Wisdom", the owl of Minerva."