This study is focused on understanding the neurophysiology of major depressive disorder (MDD), and the impact of selective serotonin reuptake inhibitors (SSRIs) in children and adolescents. This is a cross-sectional study which will utilize single and paired-pulse transcranial magnetic stimulation (TMS) to collect measures of glutamatergic cortical excitability (the motor threshold and intracortical facilitation), and GABAergic cortical inhibition (the cortical silent period and intracortical inhibition) of the motor cortex in chilren and adolescents in various disease states of MDD. The optional proton magnetic resonance spectroscopy and imaging scans (MRS/MRI) at 3 Tesla (3T) will examine glutamate concentrations in the motor cortex and anterior cingulate cortex.