**1. Introduction**

Alginate is a natural heteropolysaccharide extracted from brown seaweed. Due to the abundantly available in nature and less expensive, *L. hyperborea*, *L. digitata*, Laminaria japonica, *A. nodosum*, and *M. pyrifera* are the main choice of alginate [1]. Alginate, without pungent smell, usually has a white or yellowish-brown character. It is water-soluble but insoluble in organic solvents, such as alcohol, chloroform. Carboxylic acid groups in the saccharide residue endow the special anionic nature. Calcium alginate, for instance, is insoluble, while sodium alginate has water-solution [2]. However, the process of water dissolving is slow and usually takes several hours, forming stable and viscous solutions [3]. Due to the ability to form a hydrogel, alginate could be utilized as a gelling agent, which expands its application in the field of biomedical applications.

Alginate and its derivatives show outstanding properties, such as biocompatibility, biodegradation, gel-forming ability, being suitable for sterilization and storage. Owing to its unique characteristics, alginate has been widely used in diverse fields, including wound dressings, drug delivery, tissue regeneration. In this chapter, the characteristics and wide applications of alginate will be introduced.
