Agar-agar is a hydrocolloid extracted from red seaweeds that is widely used as a gelling  agent in the food industry. In its gelling power, agar is outstanding among the hydrocolloids. Among its major properties one can mention its high gel strength at low concentrations, low viscosity in solution, high transparency in solution, thermo-reversible gel and sharp melting/setting temperatures. Agar-agar may come in several forms: powdered, flakes, bars and threads. Besides its use as a food additive, it is also used on a lesser scale in other industrial applications.

Agar-agar is extracted from several types and species of red seaweeds belonging to the Rhodophyceae class. These agar-containing seaweeds are called agarophytes and the major commercial species are Gracilaria and Gelidium. The agar content of seaweeds varies according to the conditions of seawater. Carbon dioxide concentration, oxygen tension, water temperature and intensity of solar radiation can have significant influence.

Seaweeds are usually harvested manually by fishermen in low depths at low tides or by diving using appropriate equipment. After being harvested, seaweeds are placed under the sun to dry until they reach a humidity level that is ideal for processing. Gelidium is obtained from natural seaweed beds mainly in Morocco, Spain, Portugal, Japan and South Korea, as attempts to cultivate it have not been successful. On the other hand, Gracilaria seaweeds have been successfully cultivated on a commercial scale, particularly in China, Indonesia and Chile.