This chapter discusses the characteristics of the diverse polyphyletic group of organisms known as the algae. They range from single cells to multicellular organisms over 75 meters in length. They are found in oceans and freshwater environments and are the major producers of oxygen and organic material. A few algae live in moist soil and other terrestrial environments. They do not constitute a unique kingdom. An overview of their characteristics is presented, followed by discussion of each of the major groups of algae.