The maintenance of the local and systemic homeostasis in the human body depends on several specific and non-specific protective mechanisms. An important and one of the oldest elements of the innate, non-specific immunity in multicellular organisms are the defensins – very active, antimicrobial peptides. This review focuses on the structure and the properties of α, β and θ defensins and their biological role with a special emphasis on the oral environment. Antibacterial, antiviral and antifungal activity of the defensins, their influence on the immune response modulation in the presence of certain pathogens and the impact of their variable expression on the development of systemic diseases are discussed in this paper. We also presented the defensins’ expression sites in the oral cavity, physiologic and pathologic conditions, where the modulated defensins’ expression plays an important role and the potential options for the defensin use in the treatment of selected oral cavity diseases.