The biosynthesis of peptidoglycan is a two-stage process. The first stage concerns the endocellular assembly of its monomer unit, whereas the second one concerns the exocellular polymerization steps. The continued interest for this system is due to (i) the emergence of new resistance mechanisms; (ii) the need of specific targets in the search for new antibacterials; and (iii) the steady progress in the study of the correlation of peptidoglycan metabolism with cell growth and division. The various steps of the assembly of the monomer unit will be discussed as well as the correlations between the two stages. Finally, the flexibility of the pathway will be exemplified in Escherichia coli and Staphylococcus aureus.