Seaweeds or macroalgae are generally grouped into three taxa: green, brown, and red algae. (A more comprehensive review of seaweed systematics can be found in chapter: Macroalgae Systematics.) They are important bioresources of the oceans and play a major role in the maintenance of its ecosystem. These marine algae are utilized for food, fodder, polysaccharide extraction, biofertilizers, cosmetics, papermaking, and biobased fuels. In this chapter, numerous topics on seaweed biology are covered including: the role in maintenance of ocean ecology, herbivory and predation defense mechanisms, life history of important seaweeds, and their commercial applications.