Bacteriocins are proteinaceous compounds, which are ribosomally synthesized antimicrobial peptides produced by both Gram-positive and Gram-negative bacteria. They are phenomenologically analogous to yeast and paramecium killing factors, and are structurally, functionally, and ecologically diverse. Bacteriocins differ from the traditional antibiotics in one critical way. They have a relatively narrow killing spectrum and are toxic only to bacteria closely related to the producing strains. The bacterial membranes are the target for bacteriocins activity. Bacteriocins can be classified into several groups in which classes I and II are well studied. Bacteriocins have been used as biotechnological tools for therapeutic and commercial applications due to their specific modes of actions. Applications of bacteriocins include treatment of infectious diseases of both humans and plants and as preservatives in foods, pharmaceuticals, cosmetics and various biomedical products.