Gesso is a medium used for surface preparation.

Gesso seals the surface as well as providing a smooth, 'toothy' surface on which paint or paper will adhere to easily.

Gesso is most commonly used when creating an Oil Painting or an Acrylic Painting, as it seals the canvas surface so that it is not absorbent. Gesso can also be used in a similar way as size; a similar purpose product used for for gilding and decoupage.

There are two kinds of Gesso:
 

• Traditional Gesso - Traditionally Gesso is a material made from hide glue, mixed with powered chalk as a preparation for wood panels it sealed the surface of the wood primarily so that wood tannins didn't seep through and stain the true color of the artists paint.
   
• Acrylic Gesso: Today, Acrylic Gesso is the most common form of gesso you will find, it is a modern adaptation that shares the same name. Acrylic gesso is normally a mixture of acrylic resin and Titanium White. Some of the best brands also contain calcite which is a modern synthetic chalk.

Gesso is made by Paint Manufacturers and can be purchased in tubs of varying volumes at any art supply store.