Plant defensins form one of the classes of a large family of PR-proteins (pathogenesis-related proteins), which have different structure-functional properties and provide plant survival and adaptation under stress conditions. The defensin synthesis is activated at certain stages of the plant ontogeny, as well as during infection and under the influence of abiotic stress factors. The plant defensins exhibit a broad spectrum of biological activities, including inhibition of a growth of pathogenic microorganisms and parasitic plants, protein biosynthesis, proliferation of tumor cells, proteolytic enzymes, α-amylase, and the HIV-1 reverse transcriptase and blockage of the ion channels. Some of these peptides exhibit allergenic properties. Plant defensins are not only multifunctional peptides which participate in various processes in plants, but also important components of the immune system. This review summarizes data on the structural features of plant defensins and defensin-like proteins, signaling pathways of an activation of their biosynthesis in a plant cell, their biological functions, and their role in the immune response. Possible areas of practical application of these peptides are discussed.