Macroalgae is a collective term used for seaweeds and other benthic marine algae that are generally visible to the naked eye. Larger macroalgae are also referred to as seaweeds. The macroalgae of the Great Barrier Reef (GBR) are a very diverse and complex assemblage of species and forms. They occupy a wide variety of habitats, including shallow and deep coral reefs, deep inter-reef areas, sandy bottoms, seagrass beds, mangroves roots, and rocky intertidal zones. Macroalgae broadly comprise species from three different phyla: Rhodophyta (red algae), Heterokontophyta (predominantly Phaeophyceae, the brown algae), and Chlorophyta (the green algae) (Table 7.1). Macroalgae are clearly distinguished from microalgae, which require a microscope to be observed (eg phytoplankton, benthic and pelagic diatoms, free-living dinoflagellates, cyanobacteria (blue-green algae) and the symbiotic zooxanthellae that live within coral tissue). In some cases, benthic microalgae, such as some cyanobacteria and Chrysophyta, form large colonies that resemble thalli of macroalgae. Such colony-forming cyanobacteria are often common components of turf algal assemblages and, in this context, will be included in this chapter.