Toxins are non-replicating agents of biological origin. They are non-infectious, non-contagious, and non-curable by antibiotics or chemotherapeutic agents. However, individuals can be protected by vaccination. The multifactorial nature of virulence of toxin and toxin producers, produces comparative and cooperative pathogenesis, and this makes studies all the more difficult. Antibody raised against all components helps in this pursuit. The toxins have been classified into seven different classes and over 44 bacterial toxins have been discussed. The botulinum toxin is by far the most toxic substance in the world. All the toxins produced are either secreted out, called exotoxins (proteins), or are entrapped in the cell membrane, called endotoxins (lipopolysaccharides). These toxins are di-chain molecules, internalized into the cell by receptor mediated endocytosis, and ADP-ribosylation is the most common mode of action. The toxins produced by bacteria are enterotoxins, neurotoxins, cytotoxins, lysins (e.g., hemolysin), gangrene producing toxins. However, a single bacteria often produces more than one toxin. Bacterial toxins, which are primarily harmful, are also being used for the cure of cancer, killing of mosquito larvae, understanding of basic sciences like ADP-ribosylation, etc.