Plant defensins are small basic peptides that are inhibitory against a range of plant and human pathogens. Their in vitro antimicrobial activity and structural similarity with human and insect defensins indicated an important role for plant defensins in the innate immune system of plants. Regarding their mode of antimicrobial action, most plant defensins interact with a specific microbial surface receptor, resulting in microbial cell death via e.g. induction of apoptosis. However, accumulating evidence suggests additional in vivo functions of these plant defensins, and by extension of the more recently discovered defensin-like peptides, in general plant development. In this review we will discuss both, the functional roles of defensins in the plant and their modes of antimicrobial action.