Botulinum toxin injection for treatment of facial wrinkles is the most frequently performed cosmetic procedure in the United States, and it is one of the most common entry procedures for clinicians seeking to incorporate aesthetic treatments into their practice. Treatment of frown lines and crow's feet, which are the cosmetic indications approved by the U.S. Food and Drug Administration, and horizontal forehead lines, offers predictable results, has few adverse effects, and is associated with high patient satisfaction. Wrinkles are formed by dermal atrophy and repetitive contraction of underlying facial musculature. Botulinum toxin is a potent neurotoxin that inhibits release of acetylcholine at the neuromuscular junction. Injection of small quantities of botulinum toxin into specific overactive muscles causes localized muscle relaxation that smooths the overlying skin and reduces wrinkles. Botulinum toxin effects take about two weeks to fully develop and last three to four months. Dynamic wrinkles, seen during muscle contraction, yield more dramatic results than static wrinkles, which are visible at rest. Botulinum toxin injection is contraindicated in persons with keloidal scarring, neuromuscular disorders (e.g., myasthenia gravis), allergies to constituents of botulinum toxin products, and body dysmorphic disorder. Minor bruising can occur with botulinum toxin injection. Temporary blepharoptosis and eyebrow ptosis are rare complications that are technique-dependent; incidence declines as injector skill improves.