Glucose transporter type 1 (GLUT1) deficiency syndrome (GLUT1DS, OMIM 606777) is caused by impaired glucose transport into brain mediated by GLUT1, the glucose transporter at the blood–brain barrier. The condition is diagnosed by hypoglycorrhachia, impaired glucose uptake into erythrocytes, and heterozygous mutations in the SLC2A1 gene (OMIM 138140, gene map locus 1p35-31.3). Patients present with early-onset epilepsy, developmental delay, and a complex movement disorder. The phenotype is highly variable and several atypical variants have been described. The ketogenic diet (KD) provides ketones as an alternative fuel to the brain. Calculation, administration, supplements, and adverse effects of the KD in GLUT1DS do not differ from patients treated for intractable childhood epilepsy. In GLUT1DS, the KD should be introduced early to meet the energy demands of the developing brain and should be maintained into puberty. Seizures are effectively controlled, but the effects on neurodevelopment and on the movement disorder are less impressive. The KD remains the treatment of choice for GLUT1DS, but recent insights into anticonvulsive diet mechanisms, animal models for GLUT1DS, and the development of alternative KDs provide new opportunities to improve the treatment of this condition.