Antimicrobial peptides (AMPs) have a broad antimicrobial spectrum (gram-positive and gram-negative bacteria, fungi, certain viruses) and lyse microbial cells by interaction with biomembranes. AMPs are produced by bacteria and by other species of all kingdoms. Recently, it has been recognized that their function is essential to the immune response. AMPs participate primarily in the innate immune system of many organisms, including plants, insects and vertebrates. They have multiple roles as mediators of inflammation with impact on epithelial and inflammatory cells, influencing diverse processes such as cell proliferation, wound healing, cytokine release, chemotaxis, immune induction. Recently, pharmaceutical companies have started research programmes related to therapeutical usefulness of AMPs. This review summarises the current knowledge about the basic and applied biology of AMPs.