Gesso is an art supply used as surface preparation or primer for painting, gilding, and sculpting. Its origins are uncertain, but gesso is believed to have been developed in Italy, since the word gesso is Italian for 'chalk'. Preparation varies according to intended use, but usually consists of mixing glue with plaster, chalk, or gypsum.

Gesso resembles paint, but is thinner and dries hard. Gesso is applied with a brush and must dry before the surface can be painted. It was first created for use in painting, in order to give the surface the right properties to receive paint. In Gothic and Renaissance panel painting, gesso was applied over a panel of wood in order to give the paint something to adhere to. It created a slightly rough surface and prevented the paint from seeping into the wood.

Traditional gesso was equal volumes filler or chalk dust, white pigment either from the powdered chalk or another mineral such as zinc, and animal-skin glue, which were then heated and stirred. However, this gesso is brittle when dry and therefore susceptible to cracking. In 1955, an acrylic paint company called Liquitex developed the first water-based acrylic gesso. Modern gesso is a mixture of calcium carbonate with a pigment and an acrylic polymer medium. The pigment is usually titanium dioxide or titanium white.

Modern gesso retains the absorbent qualities of older gesso but is more flexible, and can therefore be used on canvas. It can also be colored during the manufacturing process by replacing the titanium white with another pigment. The artist can also color the gesso using watercolor, acrylic paint, or another coloring agent, in order to tint the surface to be painted. Canvases with gesso already applied are available commercially.

Gesso is also used in sculpture. In the 18th century, gesso was commonly used as a base for decorative gilding or otherwise embellishing carved woodwork such as picture frames or furniture. Gesso is not always attached to or painted over another surface; sometimes it is used directly to form the actual artwork. Gesso can be cast in a mold or used to make the mold itself, modeled, or carved. It is useful for molding or building up into relief designs. Gesso is also used in manuscript illumination, since it forms a raised area on the page that can then be gilded and burnished.

Some artists question whether modern gesso should be used under oil paint on canvas. Some materials used in oil painting, such as mineral spirits, can leak oil through the coat of gesso and damage the underlying canvas. Because of its relative newness, the archival properties of acrylic gesso are unknown.