The phenylpropanoid pathway controls the synthesis of lignin, flower pigments, signalling molecules, and a large spectrum of compounds involved in plant defence against pathogens and UV light. More than 15 P450-dependent reactions have been characterised in this pathway. Several of these reactions constitute important regulatory branching points. Indirect and direct data indicate that distinct P450s catalyse the different reactions. The coding sequences of two enzymes have recently been determined. They belong to different P450 families, CYP73 and CYP75. The second enzyme of the main pathway, cinnamate 4-hydroxylase (C4H), is the most extensively studied plant P450 and is suspected of playing an essential role in the regulation of the whole pathway.