Ongoing research concerns the non-ribosomal synthesis of natural peptide products, in particular cyclodipeptides and their complex derivatives, diketopiperazines (DKPs). DKPs constitute a large class of natural products synthesized essentially by microorganisms. Their physiological roles are still poorly understood, but some DKPs are thought to be involved in cellular communication processes. Their pharmacological activities are often remarkable and diverse, such as antibacterial, antiviral or anticancer activities (Figure 1).