Macroalgae are considerably less diverse at polar latitudes than in most temperate and tropical regions but they can still be very important components of benthic marine communities in polar waters (Dayton 1990; Wiencke et al. 2007). In fact, at some locations they can rival the biomass present in temperate kelp forests (e.g., Amsler et al. 1995). Wiencke et al. (2007) recently reviewed the ecophysiology and ecology of polar macroalgae and provided a brief overview of the state of our knowledge about their chemical defenses. The present chapter significantly expands upon that overview. Previous in-depth reviews of Antarctic marine chemical ecology (Amsler et al. 2001a, b) included macroalgae, but our knowledge of Antarctic macroalgal chemical ecology, although still relatively sparse when compared with lower latitudes, has expanded greatly since 2001. Indeed, of the 14 published or unpublished studies of polar macroalgal chemical defenses featured herein, only 3 were completed before 2001. Likewise, potential ecological roles of some Antarctic macroalgal secondary metabolites have been determined in recent years (Ankisetty et al. 2004; Lebar et al. 2007), but there are still relatively few secondary metabolites known from polar macroalgae. Not counting mycosporine-like amino acids (MAAs; see Chap. 13) or volatile halogenated organic compounds (VHOCs) (see Chap. 12), both of which include specific compounds produced by a wide diversity of algae, there are only 64 macroalgal secondary metabolites known from Antarctica and none from the Arctic Ocean (other than a presumption of the presence of phlorotannins in Arctic brown algae; Amsler et al. 2001a; Blunt et al. 2006; Lebar et al. 2007). There are, however, 18 macroalgal secondary metabolites known from northern areas of the Atlantic and Pacific Oceans that experience ice cover during some times of the year (Blunt et al. 2006; Lebar et al. 2007).