Anionic antimicrobial peptides / proteins (AAMPs) were first reported in the early 1980s and since then, have been established as an important part of the innate immune systems of vertebrates, invertebrates and plants. These peptides are active against bacteria, fungi, viruses and pests such as insects. AAMPs may be induced or expressed constitutively and in some cases, antimicrobial activity appears to be a secondary role for these peptides with other biological activities constituting their primary role. Structural characterization shows AAMPs to generally range in net charge from -1 to -7 and in length from 5 residues to circa 70 residues and for a number of these peptides, post-translational modifications are essential for antimicrobial activity. Membrane interaction appears key to the antimicrobial function of AAMPs and to facilitate these interactions, these peptides generally adopt amphiphilic structures. These architectures vary from the α-helical peptides of some amphibians to the cyclic cystine knot structures observed in some plant proteins. Some AAMPs appear to use metal ions to form cationic salt bridges with negatively charged components of microbial membranes, thereby facilitating interaction with their target organisms, but in many cases, the mechanisms underlying the antimicrobial action of these peptides are unclear or have not been elucidated. Here, we present an overview on current research into AAMPs, which suggests that these peptides are an untapped source of putative antimicrobial agents with novel mechanisms of action and possess potential for application in the medical and biotechnological arenas.