Xenobiotic metabolism in plants resembles that in animal liver on the levels of metabolite patterns, enzyme classes and cDNA sequences. Historical and current evidence for the 'green liver' concept of xenobiotic plant metabolism is presented. Xenobiotic residues resulting from phase I (transformation) and phase II (conjugation) are usually stored in the plant tissue in phase III (compartmentation). The following topics are discussed: residue toxicology, metabolic modelling, enzyme inducibility and enzyme evolution.