Botulinum toxin type A is an important therapeutic agent for the treatment of movement and other disorders. As the clinical uses of botulinum toxin type A expand, it is increasingly important to understand the biochemical and pharmacological actions of this toxin, as well as those of other botulinum toxin serotypes (B-G). Botulinum neurotoxin serotypes exhibit differences in neurotoxin complex protein size, percentage of neurotoxin in the activated or nicked form, intracellular protein target, and potency. These properties differ even between preparations that contain the same botulinum toxin serotype due to variations in product formulations. As demonstrated in preclinical and clinical studies, these differences result in a unique combination of efficacy, duration of action, safety, and antigenic potential for each botulinum neurotoxin preparation.