Agar is a cell wall component of macro red algae that can be hydrolyzed by agarase. Agarases are classified into ${alpha}$-agarase (E.C. 3.2.1.158) and ${eta}$-agarase (E.C. 3.2.1.81), in accordance with their cleavage pattern, and can be grouped in the glycoside hydrolase (GH)-16, -58, -86, -96, and -118 family according to the amino acid sequences of the proteins. Many agarases and/or their genes have been detected, isolated, and recombinantly expressed from bacteria, and metagenomes have their origins in sea and terrestrial environments. Products of agarases, agarooligosaccharides and neoagarooligosaccharides, represent wide functions such as antitumor, immune stimulation, antioxidation, prebiotic, hepa-protective, antibacterial, whitening, and moisturizing effects; hence, broad applications would be possible in the food industry, cosmetics, and medical fields. In addition, agarases are also used as a tool enzyme for research. This paper reviews the sources, purifications and detection methods, and application fields of agarases. The role of agarases in agar metabolism and the function of their enzymatic products are also surveyed.