This is an open-label clinical trial of the investigational drug creatine, for augmentation treatment of female adolescents with Major Depressive Disorder (MDD) who have failed to respond to first-line treatment with Fluoxetine or Escitalopram. Widely used by high school and college athletes in the U.S., creatine is an over-the-counter nutritional supplement with annual sales of more than $200 million. Studies in animals show that creatine improves the performance of female rats in the Porsolt Forced Swim test, which is used to predict the efficacy antidepressant compounds. Human neuroimaging studies indicate that patients with MDD have abnormal levels of high-energy phosphate metabolites in brain, primarily adenosine triphosphate (ATP) and phosphocreatine. Research has also shown that creatine supplementation induces changes in these high-energy phosphate metabolites that are associated with a positive response to antidepressants. For the proposed study, ten female adolescents between the ages of 13-18 with MDD will be recruited for participation in an open-label trial of creatine. Participants with depression will have unremitted MDD despite treatment with Fluoxetine at a dose \> 40mg daily for \> 4 weeks or Escitalopram at a dose of \> 10 mg daily for \> 4 weeks. Depressed participants will be treated with oral creatine 4gm daily for eight weeks, and will continue to take Fluoxetine or Escitalopram as prescribed. In addition, ten healthy control participants with no history of psychiatric or substance abuse disorder will be recruited. No treatment will be administered to the control participants. The primary outcome measure will be the Children's Depression Rating Scale (CDRS-R), which was used in the Treatment for Adolescents with Depression Study (TADS) clinical trial (March et al., JAMA 2004; 292(7):807-20) and the Treatment of Resistant Depression in Adolescents (TORDIA) study (Brent et al., JAMA 2008;299(8):901-13). All study participants will undergo brain scans at baseline and again after six weeks, with 31-Phosphorus Magnetic Resonance Spectroscopy (31P-MRS). The brain scans will be used to measure high-energy phosphate metabolites in the Anterior Cingulate Cortex (ACC), an anatomical region of the brain that has been implicated in MDD. 31P-MRS is a non-invasive neuroimaging technique that does not expose participants to radiation. At the magnetic field strength utilized (3T), magnetic resonance imaging is FDA-approved and has no known adverse effects. The research team will use data from 31P-MRS scans to compare levels of high-energy phosphate metabolites in MDD participants vs. healthy controls. In addition, comparison of pre- and post-treatment metabolite levels will be conducted in the MDD participants. The primary hypothesis of the study is that oral creatine will show efficacy as an augmentation treatment for female adolescents with MDD whose depression has not responded to Fluoxetine or Escitalopram. Secondary hypotheses include the following: adolescents with treatment-resistant MDD will show differences at baseline from healthy controls in high-energy phosphate metabolites in the ACC; and that brain scans in depressed adolescents who respond to creatine will show normalization of high-energy phosphate metabolites in the ACC.