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.