Today the ubiquitous distribution of high technology scanning and printing equipment enables the home user to make counterfeits of high value documents. There is an ever-increasing demand for new technologies and methods to machine authenticate printed documents and safeguard their integrity. Magnetic technology has been used to add hidden information to documents including banknotes, checks, airline tickets, identification cards, and transit documents. A solution to forensics is magnetic imaging where invisible magnetic patterns or recorded information can be displayed as an image for comparison with an optical scan. The use of small, highly sensitive detectors enables high resolution scanning of magnetic documents, creating magnetic images with fine detail. Depending on the design of a document, if the magnetic image is identical to the optical image, the document may be a counterfeit. In this paper, we will address the issues related to magnetic scanning of security documents. We will present and discuss magnetic images of documents printed with magnetic inks. We will also show how magnetic imaging can provide valuable information in understanding the alteration of magnetic data in documents such as tickets, licenses with pictures, and holograms.