Security printing is the field of the printing industry that deals with the printing of items such as banknotes, passports, stock certificates , postage stamps and identity cards. The main goal of security printing is to prevent forgery or counterfeiting.

A number of technical methods are used in the security printing industry.

Special paper

Most banknotes are made of heavy paper, sometimes mixed with linen, cotton, or other textile fibres. Some countries including Romania, Mexico and Australia produce banknotes made from plastic, in order to improve wear and tear, and permit the inclusion of a small transparent window a few millimeters in size as a security feature that is very difficult to reproduce using common counterfeiting techniques.

Watermarks

A watermark is a recognizable image or pattern in paper that appears lighter when viewed by transmitted light (or darker when viewed by reflected light, atop a dark background). A watermark is made by impressing a water coated metal stamp or dandy roll onto the paper during manufacturing. Watermarks were first introduced in Bologna, Italy in 1282; as well as their use in security printing, they have also been used by papermakers to identify their product.

Intaglio printing

Intaglio is a printing technique in which the image is incised into a surface. Normally, copper or zinc plates are used, and the incisions are created by etching or engraving the image, but one may also use mezzotint. In printing, the surface is covered in ink, and then rubbed vigorously with tarlatan cloth or newspaper to remove the ink from the surface, leaving it in the incisions. A damp piece of paper is placed on top, and the plate and paper are run through a printing press that, through pressure, transfers the ink to the paper.

The very sharp printing obtained from the intaglio process is hard to imitate by other means.

Geometric lathe work

A guilloché is an ornamental pattern formed of two or more curved bands that interlace to repeat a circular design. They are made with a geometric lathe .

Microprinting

Color-changing inks

Split ink fountains

Holograms

Security threads

Magnetic ink

Serial numbers

Hidden marks and "mistakes"

Anti-copying marks

In the late twentieth century advances in computer and photocopy technology made it possible for people without sophisticated training to easily copy currency. In an attempt to prevent this, banks have sought to add filtering features to the software and hardware available to the public that senses features of currency, and then locks out the reproduction of any material with these marks. One known example of such a system is the EURion constellation.

Copy-evident paper

Many secure documents have the feature which causes a photocopy of the document to appear obviously different from the original. For example, when photocopied, most checks will display the word "VOID" (or the equivalent in another language) on the copy, even though it is absent from the original.

Fluorescent dyes

Many banknotes incorporate dyes which fluoresce under ultraviolet light. These show up as words, patterns or pictures under UV. This feature is also incorporated into other documents - eg Northern Ireland NHS prescriptions show a picture of local '8th wonder' the Giant's Causeway in UV light.

Registration of features on both sides

Banknotes are typically printed with fine alignment between the printing on each side of the note. This allows the note to be examined for this feature, and provides opportunities to unambiguously align other features of the note to the printing. Again, this is difficult to imitate accurately enough in most print shops.