Algae are photosynthetic organisms that are unicellular or multicellular. They have a lot of nutrients such as carbohydrate, lipid, protein, vitamins, minerals, pigments, and secondary metabolites. Because of that, algae have been used in various industries such as food, medicine, biofuel, and feed for animals. Algae have a lot of bioactive compounds such as alginate, xylan, fucoidan, carrageenan, flavonoids, phenolic compounds, and others. Algae have potential as an antibacterial, antiviral, antifungal, antiaging, antioxidative, anti-inflammatory, and anti-tumor agent. This article aims to give information about the chemical composition of algae and its applications.