The past decade has seen major advances in the cloning of genes encoding enzymes of plant secondary metabolism. This has been further enhanced by the recent project on the sequencing of the Arabidopsis genome. These developments provide the molecular genetic basis to address the question of the Evolution of Metabolic Pathways. This volume provides in-depth reviews of our current knowledge on the evolutionary origin of plant secondary metabolites and the enzymes involved in their biosynthesis. The chapters cover five major topics: 1. Role of secondary metabolites in evolution; 2. Evolutionary origins of polyketides and terpenes; 3. Roles of oxidative reactions in the evolution of secondary metabolism; 4. Evolutionary origin of substitution reactions: acylation, glycosylation and methylation; and 5. Biochemistry and molecular biology of brassinosteroids.