Mechanisms and Predictors of Change in App-Based Mindfulness Training for Adolescents

Inclusion Criteria: * Both genders, all ethnicities (see Section: Inclusion of Women and Minorities) * Ages 13-18 years * Written informed assent/consent from adolescent and parent/guardian * English as a first language or English fluency * Right-handed * Personal iPhone or Android smartphone * CRSQ rumination subscale score * If on psychotropic medication, must be on stable dose for at least 2 months Exclusion Criteria: * History or current diagnosis of any of the following DSM-5 disorders: schizophrenia spectrum or other psychotic disorder, bipolar disorder, substance/alcohol use disorder within the past 12 months or lifetime severe substance/alcohol use disorder. * Systemic medical or neurological illness that could impact fMRI measures of cerebral blood flow * Failure to meet standard exclusion criteria for fMRI scanning (e.g. pregnancy, claustrophobia, cardiac or neural pacemakers, surgically implanted metal devices, cochlear implants, metal objects in the body) * History of seizure disorder, or head trauma with loss of consciousness \> 2 mins * Serious or unstable medical illness (e.g., cardiovascular, hepatic, renal, respiratory, endocrine, neurologic or hematologic disease) * Participants with active suicidal ideation will be immediately referred to appropriate clinical treatment. * Current or past treatment with mindfulness-based psychotherapy (e.g., MBCT, DBT or ACT) * Exposure to in-person or app-based mindfulness/meditation course (at least 300 mins of past practice)