Aquatint is an intaglio printmaking technique, a variant of etching that produces areas of tone rather than lines. For this reason it has mostly been used in conjunction with etching, to give both lines and shaded tone. It has also been used historically to print in colour, both by printing with multiple plates in different colours, and by making monochrome prints that were then hand-coloured with watercolour. The term colour etching, frequently used in the art trade, is potentially ambiguous, but most often means one of these two options.

It has been in regular use since the later 18th century, and was most widely used between about 1770 and 1830, when it was used both for artistic prints and decorative ones. After about 1830 it lost ground to lithography and other techniques. There have been periodic revivals among artists since then. An aquatint plate wears out relatively quickly, and is less easily reworked than other intaglio plates. Many of Goya's plates were reprinted too often posthumously, giving very poor impressions.

Among the most famous prints using the aquatint technique are the major series by Goya, many of The Birds of America by John James Audubon (with the colour added by hand), and prints by Mary Cassatt printed in colour using several plates.

In intaglio printmaking techniques such as engraving and etching, the artist makes marks into the surface of the plate (in the case of aquatint, a copper or zinc plate) that are capable of holding ink. The plate is inked all over then wiped clean to leave ink only in the marks. The plate is passed through a printing press together with a sheet of paper, and strong pressure applied pushing the paper into the marks, so that a transfer of the ink to the paper occurs. This is repeated many times to produce multiple "impressions" (copies).

Like etching, aquatint uses the application of a mordant (acid) to etch into the metal plate. Where etching uses a needle to scratch through an acid-proof resist and make lines, aquatint uses powdered rosin (resin) to create a tonal effect. The rosin is acid resistant and typically adhered to the plate by controlled heating; where the grains area will print white, with black areas around. The tonal variation is controlled by the level of mordant exposure over large areas, and thus the image is shaped by large sections at a time. The rosin is then washed off the plate before printing.

Various other substances than rosin have been used, including bitumen (asphalt). The necessary qualities are that the substance can be made into a powder, will resist acid, and can be fixed to the copper plate, usually by heating. The shallowness of the areas bitten by the acid accounts for the limited life of the plates, as the depressions in the copper are gradually flattened out.

Aquatint may be the only technique used on a plate, but is very often combined with a linear technique such as etching, especially for outlines, figures and buildings.

Another tonal technique, mezzotint, begins with a plate surface that is evenly indented and roughened so that it will print as an even and fairly dark tone of ink. The mezzotint plate is then smoothed and polished to make areas carry less ink and thus print a lighter shade. Alternatively, beginning with a smooth plate, areas are roughened to make them darker. Occasionally the two techniques of aquatint and mezzotint are combined.