Plant phenolics are a structurally diverse group of compounds synthesized by all higher plants. The toxicity of phenolics and the products of their hydrolysis and oxidation has long been appreciated, and has implicated this group of natural products with plant defense. Unfortunately, much of the literature relating phenolics to plant defense is published in sources rarely consulted by evolutionists or ecologists. The purpose of this paper is to make a case for plant phenolics as defensive substances by considering their structural diversity, concentration, alteration upon infection, and mode of action. Correlations between ecogeographical and ecological parameters and phenolic profiles are discussed in light of the putative defensive role of phenolics, and additional correlations are predicted.