The degradation of lignin by filamentous fungi is a major route for the recycling of photosynthetically fixed carbon, and the oxidative mechanisms employed have potential biotechnological applications. The lignin peroxidases (LiPs), manganese peroxidases (MnPs), and closely related enzymes of white rot basidiomycetes are likely contributors to fungal ligninolysis. Many of them cleave lignin model compounds to give products consistent with those found in residual white-rotted lignin, and at least some depolymerize synthetic lignins. However, none has yet been shown to delignify intact lignocellulose in vitro. The likely reason is that the peroxidases need to act in concert with small oxidants that can penetrate lignified tissues. Recent progress in the dissolution and NMR spectroscopy of plant cell walls may allow new inferences about the nature of the oxidants involved. Furthermore, increasing knowledge about the genomes of ligninolytic fungi may help us decide whether any of the peroxidases has an essential role.