In the 2 decades since the approval of botulinum toxin for medical use in the United States, neurotoxin for facial enhancement has become a standard of care for the treatment of dynamic wrinkles. Transient paralysis of the musculature underlying facial rhytides can reduce the appearance of wrinkles. In recent years, refinement of technique and combination therapy with other cosmetic modalities such as lasers and filling agents has allowed practitioners to create a more natural appearance, rather than one that is expressionless. As more botulinum toxin products become commercially available, it is clear that differences exist and the nuances of dosing and injection pattern continue to be elucidated. Part 1 of this series will focus on the science of the currently available neurotoxin products. Part 2 will outline best practices for their use in aesthetic dermatology.