Cerebral creatine deficiency syndromes are a group of genetic disorders caused by the inability to synthesize or transport creatine, leading to a severe depletion of creatine in the brain. Creatine transporter deficiency is one of the three Cerebral creatine deficiency syndromes caused by loss of function mutations in the X-linked SLC6A8 gene. In the absence of creatine transporter cells are unable to uptake creatine, resulting in a depletion of creatine in multiple tissues, including the brain. Symptoms include intellectual disability, developmental delays, hyperactivity, and motor function deficits. As creatine is unable to cross cell membranes, supplementation is ineffective, and there are no currently available treatments. Here, we present the development of a gene therapy for creatine transporter deficiency using a self-complementary adeno-associated virus serotype-9 vector. We first attempted to replicate previously reported phenotypes in a mouse model of creatine transporter disorder to use as a measure of gene therapy efficacy. We were unable to replicate deficits in memory but were able to show increased hyperactivity and decreased brain creatine. In Chapter 4. We show that gene therapy is capable of restoring creatine in creatine transporter-deficient patient cells. Then, in a proof of concept, gene therapy following neonatal intracerebroventricular (ICV) administration increases brain and peripheral organ creatine in a dose-dependent fashion. Brain creatine was able to be elevated to supraphysiological levels in creatine transporter knockout mice, indicating that gene therapy holds the potential for complete restoration of the biochemical phenotype. As neonatal gene therapy is difficult to translate to the clinic, we next compared neonatal ICV delivery with intrathecal lumbar puncture (IT) delivery in adult mice, as IT is much more clinically relevant. Both routes were assessed at 13 and 24-week endpoints. ICV resulted in a greater increase in brain creatine than IT at both endpoints. However, IT administration still resulted in increases in brain creatine, improvement in body mass, and a reduction in hyperactivity. This shows the first evidence of a clinically feasible gene therapy in treating creatine transporter deficiency.