Algal polysaccharides (phycocolloids), such as agar, alginates, and carrageenans, are produced in a large scale (Bixler and Porse 2011) and have a wide range of applications in food, pharmaceutical, and cosmetic industries. This chapter focuses on seaweed-derived hydrocolloids, with special attention to carrageenan and its extraction, characterization, and applications. Water-in-oil emulsions such as mayonnaise and salad dressings are less likely to separate into their original oil and water phases if thickened with alginate. IR spectroscopy has been applied for many years in the characterization of sulfated polysaccharides from seaweeds, and it was, until recently, the most frequently used vibrational technique for studying the natural products. The industrial interest and economic importance of carrageenans are due to their ability to increase the viscosity of solutions or to form thermo-reversible gels. Acid and oxidative agents can hydrolyze carrageenan solutions, leading to loss of their physical properties due to the cleavage of glycosidic bonds.