By far the more important and versatile printing process today is offset lithography. The underlying principles were established at the end of the 18^th century by a German map inspector, Aloys Senefelder, who was experimenting with methods of producing limestone relief printing surfaces using an acid etching process. Senefelder found that a wet limestone surface would repel an oil-based printing ink, and that an image drawn on the surface with a grease pencil would repel water and attract ink. Bringing a damp sheet of paper into contact with the freshly inked image could reproduce any drawing on the stone surface. This cycle could be repeated several hundred times before the drawing could no longer be faithfully reproduced.

The process, called chemical printing by Senefelder, quickly became a popular art medium because it enabled artists to produce multiple copies of freehand drawings. By the late 19^th century, multiple stones were being used to transfer as many as 30 separate colors to a single sheet of paper to produce exquisite color lithographs that resembled fine watercolor paintings. Modern color lithography uses only four inks for a wide range of natural colors.

In the early part of the 20thy century, it was discovered that ink could be transferred from the lithographic surface to an intermediate rubber surface and then to paper. The rubber intermediate, called a blanket, can transfer ink to paper and to a wide variety of materials that cannot be printed directly, including plastics and metals. Because the soft blanket conforms to the texture of the surface to be printed, lithographic image quality is unrivaled.

The function of the original stone printing surface is now served by thin aluminum plated, although other materials, such as stainless steel and plastics, can also be used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber rollers carry ink and water to the plate surface. The ink is transferred first to the blanket cylinder and then to the paper.

Lithographic plates are the least expensive printing surfaces available today, and this fact has contributed greatly to the success of the process. Aluminum plate materials have a thin surface coating of light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an intense source of blue and ultraviolet light. Images are transferred to the surface by exposing the plate through a film positive or negative. Some materials can be exposed directly, as in a graphic-arts camera or by a computer-controlled laser beam. Thereby eliminating the expense of film and speeding up the platemaking process.

Modern offset lithographic presses range in six from small sheet-fed duplicators—used for small, single-color job such as brochures and newsletters—to massive web presses capable of printing millions of copies of magazines, catalogs, mailing pieces, and packaging materials in full color. No other process has such a broad range of applications.