This book is concerned with discovering the chemical pathways of biosynthesis. The four chapters cover the use of isotopes in biosynthetic research and the biosynthesis of enzyme cofactors and vitamin B12 and of reduced polyketides such as erythromycin. The topics covered demonstrated the revolution that has occurred in biosynthetic studies with the advent of gene cloning and overexpression. The knowledge of biosynthetic pathways at the genetic and enzymic levels which is becoming available opens up possibilities of designing inhibitors of particular steps as new antibiotics or manipulating the genes so that new natural products are produced. However the book also shows that the more classical approach to biosynthetic studies must go hand in hand with these new techniques.