Botulinum neurotoxins are produced by anaerobic spore-forming bacteria of the genus Clostridium in several dozens of variants that inactivate neurotransmitter release owing to their metalloprotease activity. This results in a persistent paralysis of peripheral nerve terminals known as botulism. They are the most potent toxins known and are classified as one of the six highest-risk threat agents of bioterrorism. Despite their high toxicity, two of them are used as valuable pharmaceutical for the therapy of many neurological and non-neurological disorders. Notwithstanding the many advances in our understanding of the genetics and structure of botulinum neurotoxins, there are still many gaps in knowledge of toxin mechanism of action that will be discussed here.