First used anecdotally for the control of eclamptic seizures in the early 1900s, magnesium sulfate remains 1 of the most commonly used medications in obstetric practice today. Over the past 95 years, there have been countless research studies investigating the efficacy of magnesium sulfate for the management of eclampsia, preeclampsia, preterm labor, and most recently for prevention of cerebral palsy. The majority of this evidence undeniably supports the use of magnesium sulfate as the drug of choice for the prevention and treatment of eclampsia when evidence of severe disease is present. On the other hand, studies have not shown magnesium sulfate to be comparably more effective than other tocolytics for treating preterm labor, nor is there agreement on whether the evidence supports its use as a neuroprotective agent for prevention of cerebral palsy. The exact mechanism of action of magnesium sulfate remains largely hypothetical, and parenteral use has the potential to cause significant morbidity in high doses. This article reviews magnesium sulfate's remarkable history in obstetric practice and includes a summary of the evidence related to each of the controversies. An initial review of the physiology of magnesium sulfate is essential to understanding pharmacodynamic actions, dosing guidelines, and safety requirements.