The "I Want to Believe" poster is the name given to a series of posters that share a similar layout and could often be found on the wall of Fox Mulder's offices. (The X-Files, The X-Files: I Want to Believe).
Mulder originally acquired his "I Want to Believe" poster from a head shop on M Street. (TXF: "Chinga") He had obtained the poster and attached it to the wall of his basement X-Files office shortly prior to March 1992. This first poster consisted of a flat designed UFO above a field of trees inside of a white border that had the tag line "I WANT TO BELIEVE" written on it. (TXF: "Pilot") This poster was destroyed by a fire in the office in 1998. (TXF: "The End")
However, Mulder acquired a replacement poster once more stating "I WANT TO BELIEVE". This Second poster was almost identical in set up to his previous one and he attached it to the same wall as its predecessor. This copy of the poster however had a fatter UFO positioned slightly differently to the original version as being located above a different set of trees. (TXF: "Alpha") This poster was later swapped out with a third version that was identical to the second but included a UFO which more closely resembled the first poster.
In 2002, this poster was the only thing not cleared out of the office but was left lying on the room's floor until Agent Doggett rolled it up and brought it out of the room. (TXF: "The Truth") This copy of the poster was later torn when Mulder kicked it while it was laying on the floor after it was presumably returned to the office by Agent Doggett or another person. (TXF: "My Struggle") In 2008, Mulder had a fourth version of the poster to the wall of his home office in rural Virginia. This design of the poster was identical to its third copy, though it lacked the white border seen on the other three. (The X-Files: I Want to Believe).
In 2016, a fifth version of the poster can yet again be seen on the wall of Mulder's office in Virginia. (TXF: "My Struggle") This version has the same layout to all 4 prior, though the UFO is instead positioned above a group of red trees. In the same year, yet another poster was later seen in Mulder and Scully's office at the FBI. This poster appeared to be completely identical to the third version but unlike the others, Scully says that this one was her poster instead of Mulder's. This poster had multiple holes in it from Mulder throwing his pencils at it, similar to his old pass time of throwing pencils at the ceiling. (TXF: "Mulder & Scully Meet the Were-Monster)