Interactions between macrophytes and marine herbivores play central roles in regulating and structuring nearshore communities, biodiversity, and their cycling of nutrients and materials, and can determine the success of human introductions and poleward expansion of seaweeds due to a warming ocean. In this chapter, we outline our current understanding of herbivore offenses, with a focus principally on their responses to algal chemical defenses. We briefly outline the spatial and temporal variation in seaweed defenses, highlight the range of herbivore responses to seaweed defenses, and review the evidence for their genetic basis and ecological constraints on herbivore responses. We end with a review of micro- and macroevolutionary patterns in herbivore offenses documented to date.