Host defence peptides (HDPs) are polypeptide sequences found ubiquitously in nature that have garnered significant attention as alternatives to antibiotics. Originally appreciated for their direct antibacterial effect, recent work has revealed that many HDPs possess antibiofilm activity, anticancer activity and/or the ability to modulate the immune response of the host. Curiously, HDPs with potent activity in one arena are often weakly active with respect to other activities described for this class of molecules. In principle, synthetic HDPs could be optimized for specific biological purposes provided sufficient sequence information were available to predict the activity of novel peptides. Unfortunately, beyond the generic properties of HDPs (eg. positive charge and amphipathicity), our current understanding of the peptide sequence requirements that govern these diverse biological effects is limited, particularly for the immunomodulatory and antibiofilm properties. In this seminar, I will highlight some of the peptide screening and design strategies that we have used to explore the sequence requirements that contribute to these various peptide-mediated activities and demonstrate that peptides can be engineered for specific biological applications. In addition, I will discuss the incorporation of non-natural amino acids and peptidomimetics into synthetic HDP sequences to improve stability while retaining crucial biological functions. Overall, our work improves our understanding of the sequence requirements of HDPs and paves the way for synthetic HDPs that are specifically designed for a diverse range of therapeutic applications.