Indocyanine green (ICG) is a tricarbocyanine organic dye that has diverse clinical uses including cardiac dye-dilution studies, liver function and blood flow determination, and ophthalmic angiography. This chapter presents a case study of a 67-year-old American Society of Anesthesiologists Class III female scheduled to undergo elective left pterional craniotomy for clipping of intracranial aneurysms. Adverse reactions to ICG dye vary both in system involvement and severity. Treatment in case reports has included intravenous crystalloid and colloids, airway management if necessary, corticosteroids, epinephrine, diphenhydramine, beta-agonist nebulizers, and theophylline. It was initially proposed that patients with iodine sensitivity were susceptible because of the solubilizing iodine component of the pharmaceutical product, but this has been refuted by a large case series. Both anaphylactoid and nonallergic reactions have been proposed as possible mechanisms for ICG dye reactions. Awareness of adverse reactions associated with ICG dye is imperative given its increasing use in neurosurgery.