Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.

It is resistant to weathering and slumping compared to the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.

Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is normally formed underwater, commonly on the sea bed, then consolidated and compressed during diagenesis into the form commonly seen today. During diagenesis silica accumulates to form chert or flint nodules within the carbonate rock.

In agriculture, Chalk is used for raising pH in soils with high acidity.

In field sports, including grass tennis courts, powdered chalk is used to mark the boundary lines of the playing field or court. This gives the advantage that, if the ball hits the line, a cloud of chalk or pigment dust can be seen.

In gymnastics, rock-climbing, weight-lifting and tug of war, Chalk is applied to the hands to remove perspiration and reduce slipping.

Tailor's Chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors.

Toothpaste also commonly contains small amounts of chalk, to serve as a mild abrasive.