Children with cerebral palsy often have spasticity of the legs, a condition in which the legs are stiff because of involuntary muscle overactivity caused by the brain or spinal cord disorder. Spasticity causes poor coordination, spasms, abnormal posture and pain, and contributes greatly to the developmental deformities and disability of cerebral palsy. Conventional treatment with physiotherapy, splinting, oral medications and sometimes plaster casting and surgery may prove inadequate. Open label studies and some RCTs suggest that botulinum toxin injections into the spastic muscles can alleviate the spasticity and help some of these problems. Botulinum toxin blocks the release of acetylcholine from the neuromuscular junction and weakens the muscle. This and other effects may account for the apparent benefit in spasticity, but also highlight the importance of clarifying safety, especially in this group of growing children.