Chemical defenses against herbivores are common in macroalgae from Antarctica, particularly in those species that typically dominate benthic communities. Conversely, although little studied, chemical defenses do not appear to be as important in the ecological relationships of Arctic macroalgae. Mesoherbivory may be particularly heavy in Antarctic macroalgal communities, but Antarctic macroalgae produce chemical defenses against both mesoherbivores and macroherbivores. Antarctic macroalgae have been useful in testing and extending chemical defense theories. While physical defenses may also be important in some Antarctic macroalgal-herbivore relationships, particularly with respect to mesoherbivores, physical defenses do not appear to have wide-spread importance against macroherbivores. In Arctic macroalgae, physical defenses may be of more overall importance in reducing mesoherbivory. Antarctic macroalgae also produce compounds with the potential to control biofouling by sympatric microalgae. © 2009 by Walter de Gruyter.