Publisher Summary This chapter discusses the biosynthesis of cephalosporins. A variety of cephalosporins and their producers has been found, however it has not been conclusively determined what the secondary metabolites are in a strict sense of the term. A moderate number of the metabolites are surmised in this chapter to be intermediates of the biosynthesis and degraded or modified products of secondary metabolites. It summarizes Cephalosporins and various metabolites related to the biosynthesis of β-lactam antibiotics in cephalosporin-producing strains. The features of the biosynthetic pathway of the β -lactam antibiotics, and especially of cephalosporins, have been disclosed through extensive work by several groups. The hypothesis that the tripeptide is a direct precursor of the β-lactam antibiotics was supported by the experiments of Fawcett et al. The most important problem about β-lactam antibiotic synthesis now involves the mechanism of the ring closure, i.e., the ability of the oxidation mechanism of the valine moiety to form the five-membered or the six-membered ring and of the cysteine moiety to form the β-lactam ring.