Inclusion Criteria:  Be18-64 years old; Able and willing to provide informed consent; Have moderate-to-severe depressive symptoms, with a PHQ-9 score ≥ 10; Insufficient moderate-to-vigorous physical activity (< 90 minutes a week); Demonstrated interest in increasing physical activity; Have a smartphone.  Exclusion Criteria:  Have any current, past, or lifetime manic or hypomanic episode, psychosis, schizophrenia or schizophreniform disorder; Be currently experiencing active suicidal ideation (i.e. with suicidal thoughts, plan, and intent) or at a high risk for suicide during the course of the study, as designated by the PI; Have a medical condition that prohibits physical activity; be medically prohibited to exercise by primary care physician (PCP), OB-GYN (obstetrician-gynecologist) or study MD. Be in current, active psychotherapy