Algae are a large and diverse group of eukaryotic (complex-celled) photosynthetic organisms. The green algae are primitive members of kingdom Plantae, though other types of algae are outside the kingdom entirely. Land plants evolved from green algae about 500 million years ago. The algae are a paraphyletic group, meaning they did not descend from a common ancestor. Algal groups have independently evolved several times in life on Earth, representing a beneficial strategy adopted by parallel evolutionary paths.

Red algae, a primitive form, were among the first eukaryotic organisms on the planet, and their signatures have been found in rocks 1.2 billion years old, but they are likely older. All forms of algae evolved from cyanobacteria, photosynthetic bacteria that were among the first life forms on the planet. It took several billion years for bacteria to evolve into eukaryotic organisms, which are the ancestors of all living plants and animals. Cyanobacteria is called "blue-green algae" in some older texts, but today the word is usually restricted to eukaryotic organisms.

The most complex algae of both the red and green versions, are called seaweeds, and can be found washed up on beaches around the world. Algae, along with cyanobacteria, are the primary producers of the marine ecosystem, and are consumed by a wide variety of organisms, especially filter feeders. Most photosynthesis in the oceans takes place in the upper 100 m (328 ft). Below this, the water gets progressively darker until it is pitch black, and no photosynthesis can occur. Those animals that live deep in the oceans depend on dead algae or small animals floating down from above.

Algae may be similar to protozoa, that is, microscopic mobile organisms, differentiated by their ability to photosynthesize. Like land plants, most groups of algae are usually stationary, and lack locomotory abilities. Like other eukaryotic photosynthesizers, the photosynthetic machinery of algae is derived from endosymbiotic cyanobacteria-like organelles called chloroplasts. Eukaryotic organisms likely evolved from extremely closely integrated communities of symbiotic bacteria. Worldwide, there are about 5,000 species of red algae, 2,000 of brown algae and 8,000 of green algae.