Mechanisms of Behavioral Activation (BA)

Inclusion Criteria: * Male and female adolescents ages 15-17; * current MDD diagnosis as determined by K-SADS-PL, * CDRS-R raw score \> 45 (T-score \> 65) at baseline; * estimated full scale IQ \> 80 as determined by the WASI-II; * able to receive outpatient care; * willing to not partake in other psychosocial treatments; * not taking psychotropic medications in the two weeks (four weeks for fluoxetine) prior to consent, with the exception of psychostimulant medication prescribed for the treatment of ADHD. * Additionally, depressed patients must have access to required technology (e.g. smart phone, tablet, or desktop computer) to complete the EMA portion of the study. * Healthy controls cannot not meet criteria for MDD and their CDRS-R raw score must be below 28; T-score \< 52). Exclusion Criteria: * Current or past diagnosis of bipolar, schizophrenia, schizophreniform, schizoaffective disorders, or psychosis NOS; * current diagnosis of developmental disorder, severe conduct disorder, life-threatening anorexia, OCD, or autism-spectrum disorders; * taking psychotropic medications prior to entry; * estimated IQ \< 80; alcohol/drug dependence or abuse within the last 3 months; * potential/confirmed neurological disorder or epilepsy; * claustrophobia; * presence of a medical condition that precludes fMRI; * endorsement of imminent and serious suicidality; * medical conditions that take precedence over the presence of MDD; * pregnancy; * substance use (excluding nicotine) within two weeks of the fMRI scan; * left-handedness; * nicotine use within 3 hours of the fMRI scan; * history of traumatic brain injury or of being unconscious for more than 30 minutes; * a stimulant regime for ADHD which has changed within 30 days of the fMRI scan. * Non-compliance (i.e. \<70% completion) or inability (i.e. lack of smartphone device) to complete the 3 assessment periods of EMA.