Lignoids, oligomeric arylpropanoids, possess allelochemical activity in plants and pharmacological activity in humans, a fact which has led to important therapeutic applications. The most diversified groups are formed by lignans, derived by the oxidative condensation of cinnamyl alcohols among themselves or with cinnamic acids, and by neolignans, derived by the oxidative condensation of allylphenols, of propenylphenols or of both. The chemistry of lignans has been well known for decades. The chemistry of neolignans was described only recently, chiefly through our own work with Amazonian plants.