Antimicrobial peptides are naturally occurring antibiotics. As part of the innate immune system of vertebrates they have direct antimicrobial function. Further, they can act as mediators of inflammation. Their antimicrobial spectrum covers gram-positive and -negative bacteria as well as fungi and certain viruses. Based on their structure, antimicrobial peptides can be divided into several families. Peptides of the defensin, cathelicidin, and histatin families have been isolated from humans, where they have been found in defense cells, such as macrophages or neutrophils, as well as in epithelial cells. Decreased production of antimicrobial peptides is associated with immune deficiencies. Further, lung disease in cystic fibrosis may be linked to the dysfunction of antimicrobial peptides. Based on naturally occurring antimicrobial peptides, derivates of these molecules were developed as innovative antibiotic drugs. The present review focuses on the biology of antimicrobial peptides as well as their potential as drugs.