Extracellular DNA (eDNA) is an important biofilm component that was recently discovered. Its presence has been initially observed in biofilms of Pseudomonas aeruginosa, Streptococcus intermedius, Streptococcus mutans, then Enterococcus faecalis and staphylococci. Autolysis is the common mechanism by which eDNA is released. In P. aeruginosa eDNA is generated by lysis of a bacterial subpopulation, under control of quorum sensing system. In E. faecalis autolysis proceeds in a fratricide mode, resulting from a process similar to necrosis of eukaryotic cells. In Staphylococcus aureus autolysis originates by an altruistic suicide, i.e., a programmed cell death similar to apoptosis of eukaryotic cells. In S. aureus autolysis is mediated by murein hydrolase, while in S. epidermidis by the autolysin protein AtlE. In P. aeruginosa eDNA is located primarily in the stalks of mushroom-shaped multicellular structures. In S. aureus the crucial role of eDNA in stabilizing biofilm is highlighted by the disgregating effect of DNase I. eDNA represents an important mechanism for horizontal gene transfer in bacteria. eDNA and other microbial structural motifs are recognized by the innate immune system via the TLR family of pattern recognition receptors (PRRs).