Tetanus and botulinum neurotoxins, produced by anaerobic bacteria of the genus Clostridium, are the most toxic proteins known and are the sole responsible for the pathogenesis of tetanus and botulism. They enter peripheral cholinergic nerve terminals and cleave proteins of the neuroexocytosis apparatus causing a persistent, but reversible, inhibition of neurotransmitter release. Botulinum neurotoxins are used in the therapy of many human syndromes caused by hyperactive cholinergic nerve terminals. Here we focus on the many advances that were recently made on the understanding of their molecular mechanism of action and on their use in human therapy.