Small peptides have powerful biological activities ranging from antibiotic to immune suppression. These peptides are synthesized by non-ribosomal peptide synthetases (NRPS). Structural understanding of NRPS took a huge leap forward in 2002; this information has led to several detailed biochemical studies and further structural studies. NRPS are complex molecular machines composed of multiple modules and each module contains several autonomously folded catalytic domains. Structural studies have largely focused on individual domains, isolated from the context of the multienzyme. Biochemical studies have looked at individual domains, isolated whole modules and intact NRPS, and the combined data begin to allow us to visualize the process of peptide assembly by NRPS.