The ecological interactions of brown algae are important as these macroalgae are common and often keystone members in many benthic marine communities. This review highlights their chemical interactions, particularly with potential herbivores, but also with fouling organisms, with potential pathogens, with each other as gametes, and with their microenvironments when they are spores. Phlorotannins, which are phenolic compounds unique to brown algae, have been studied heavily in many of these respects and are highlighted here. This includes recent controversy about their roles as defences against herbivory, as well as new understanding of their roles in primary cellular functions that may, in many instances, be more important than, and which at least have to be considered in concert with, any possible ecological functions. Brown algae have also been useful models for testing theories about the evolution of and ecological constraints on chemical defence. Furthermore, their microscopic motile gametes and spores have the ability to react to their chemical environments behaviourally. © 2005, Elsevier Inc. All rights reserved.