Defensins represent a diverse set of antimicrobial peptides found in almost all multicellular organisms. These small proteins can be characterized by an N-terminal signal sequence, a highly divergent mature protein with the exception of conserved cysteine residues, presence of defensin motifs, and a tissue-specific expression pattern. Defensin expression can be induced by pathogen inoculation and environmental stress. Until recently, defensins in plants were thought to be members of small gene families. However, the advent of expressed sequence tag (EST) and genome sequencing coupled with novel bioinformatic techniques has allowed researchers to recognize the size and diversity of the family. Recent research has identified over 300 defensin-like (DEFL) genes in each of the genomes of Arabidopsis thaliana (L.) Heynh. and Medicago truncatula Gaertner. In addition, over 1000 DEFL genes have been identified from the plant EST projects. The identification of such a broad family involved in defense against pathogens and environmental stress provides new opportunities for crop improvement. This review focuses on genome level analyses of DEFL genes in plants.