A self-verifying security document, such as a banknote, comprises a flexible sheet formed from a plastics substrate bearing indicia. The sheet has a window of transparent plastics material that includes self-verification means for verifying a security device provided at a laterally spaced second portion of the sheet when the sheet is bent or folded to bring the window into register with the security device. The self-verification means may be an optical lens for reading an area of microprinting. In another embodiment, the self-verification means may be an optical filter for viewing an area printed with metameric inks. In other embodiments, the self-verification means and the security device may be polarizing windows or Moire inducing patterns.