Offset printing is a common printing technique in which the inked image is transferred (or “offset”) from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier. Ink rollers transfer ink to the image areas of the image carrier, while a water roller applies a water-based film to the non-image areas.

The modern “web” process feeds a large reel of paper through a large press machine in several parts, typically for several meters, which then prints continuously as the paper is fed through.

Advantages of offset printing compared to other printing methods include:

• consistent high image quality. Offset printing produces sharp and clean images and type more easily than, for example, letterpress printing; this is because the rubber blanket conforms to the texture of the printing surface;
• quick and easy production of printing plates;
• longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates used with optimized inks and fountain solution may achieve run lengths of more than a million impressions;
• cost. Offset printing is the cheapest method for producing high quality prints in commercial printing quantities;
• ability to adjust the amount of ink on the fountain roller with screw keys. Most commonly, a metal blade controls the amount of ink transferred from the ink duct to the fountain roller. By adjusting the screws, the operator alters the gap between the blade and the fountain roller, increasing or decreasing the amount of ink applied to the roller in certain areas. This consequently modifies the density of the color in the respective area of the image. On older machines one adjusts the screws manually, but on modern machines the screw keys are operated electronically by the printer controlling the machine, enabling a much more precise result

The plates used in offset printing are thin, flexible, and usually larger than the paper size to be printed. Two main materials are used:

• Metal plates, usually aluminum, although sometimes they are made of multi-metal, paper, or plastic.
• Polyester plates, which are much cheaper and can be used in place of aluminum plates for smaller formats or medium quality jobs, as their dimensional stability is lower.

Offset lithography became the most popular form of commercial printing from the 1950s (“offset printing”). Substantial investment in the larger presses required for offset lithography was needed, and had an effect on the shape of the printing industry, leading to fewer, larger, printers. The change made a greatly increased use of colour printing possible, as this had previously been much more expensive. Subsequent improvements in plates, inks, and paper have further refined the technology of its superior production speed and plate durability. Today,^[when?] lithography is the primary printing technology used in the U.S. and most often as offset lithography, which is “responsible for over half of all printing using printing plates”.