Brown algae, a large and diverse group of multicellular eukaryotic and photoautotrophic organisms, play an important ecological role in carbon dioxide removal, marine carbon fixation and cycle, and rocky coastal environments. There are three main polysaccharides acting as structural components of brown algae, including alginates, sulfated fucans, and laminarin. The alginate content, which accounts for approximately 40% of the algae dry weight, fluctuates along with seasonal changes. Both alginate and its degradation product alginate oligosaccharides (AOSs) have been widely used in pharmaceutical, food, chemical, and other areas due to their high viscosity and gel properties. Alginate can stimulate the growth of human endothelial cells and promote the secretion of cytokines by macrophages. From the chemical structure aspect, alginate is a linear polysaccharide composed of two sugar units, α-L-guluronate (G) and its C5 atom epimer, β-D-mannuronate (M), which can be polymerized as three distinct blocks, a Poly-M, a Poly-G, and a Poly-MG, connected by α-1, 4-glycosidic bonds. These covalent bonds of alginate can be cleaved by alginate lyase through a β-elimination reaction, resulting in the formation of the unsaturated uronic acid with C4, 5-double bond at the non-reducing end. Besides, alginate can also be converted into AOSs by endo- and exo-alginate lyases. Therefore, alginate lyases are promising biocatalysts for producing renewable chemicals and biofuels. In this chapter, we mainly summarize the sources, classification, structural characteristics, and functions of alginate lyases and introduce their applications and future aspects.