Lumbar puncture (LP) plays a critical role in both pediatric medical diagnosis and treatment today. An accurate physical neurologic assessment will be a vital part in the medical decision making process about whether an LP should be performed. LP is used to obtain a sample of cerebro-spinal fluid (CSF) to aid in the diagnosis of infectious, inflammatory, oncologic, and metabolic processes. Therapeutic indications include the delivery of chemotherapy, antibiotics, and anesthetic agents. Increased intracranial pressure (ICP) with pseudotumor cerebri and after hemorrhage or meningitis can be effectively treated with repeated LP without shunting in some children. Diagnostic LP indications include suspicion of meningitis, in a child younger than 12 months with febrile convulsion, for measurements of antibody titers in subacute sclerosing PANENCEPHALITIS, measles, rubella, and progressive rubella PANENCEPHALITIS, suspected subarachnoid hemorrhage, pseudotumor cerebre, lead encephalopathy, diagnosis of CNS neoplasia, and some neurologic conditions (eg, normal pressure hydrocephalus, Guillain-Barre syndrome). Contraindications to LP can be absolute or relative. Increased ICP is an absolute contraindication. For patients with a bleeding diathesis or cardiopulmonary instability, the contraindications are relative to the importance of CSF results for immediate management decisions. Normal CT scan brain does not mean it is safe to do LP. Clinical signs, not a CT scan brain, are the best indicators of when to or when not to do LP. The most common complication of LP is headache. Other complications are backache or spasms in the lower back or thighs, transient voiding problems, seizures, slight rise in temperature, bleeding, shock, infection, transtentorial or cisternal herniation ("coning"), spinal/epidural abscess, epidermoid tumor, herniation of nerve roots and cerebral herniation (the most serious complication). Knowledge of the contraindications, the pertinent anatomy, and the methods to minimize the risk of complications are necessary for the safe and efficient performance of the procedure. Risks, although rare, can be substantial and even potentially life-threatening.