Accumulating evidence from the animal literature, healthy populations, and schizophrenia studies suggests that regular exercise positively affects integral functions such as neurogenesis, synaptic plasticity and cognition. Likewise, preliminary evidence suggests that aerobic activity has been associated with improved quality of life and a lower level of symptoms in patients with schizophrenia. Because exercise has been found to stimulate human medial temporal neurogenesis, and related abnormalities have been widely observed in studies of schizophrenia, physical activity may be in an important intervention. During the psychosis prodrome, a period immediately proceeding formal onset of psychotic disorders, adolescents experience subtle attenuated symptoms coupled with cognitive deterioration and a global decline in socio-occupational functioning and anywhere between 10-35% go on to transition to a psychotic disorder such as schizophrenia in a two-year period. Despite the promise of exercise interventions, and the critical role medial temporal lobe abnormalities play in etiological models of psychosis, there have been no experimental studies of aerobic exercise in ultra-high risk youth (UHR). Understanding the potential benefits of aerobic exercise in UHR youth is integral as the prodrome is a viable period of intervention in which considerable brain development is still occurring. Further, as there have been challenges associated with many of the available interventions, and an increasing level of potential found in neuroplasticity-based interventions, understanding the effect of exercise on respective brain-behavior holds considerable promise. Experimental research is sorely needed to determine if prescribed aerobic exercise can stimulate medial-temporal neurogenesis and ameliorate cognition and symptoms/functioning in this vital group. In the proposed study, an expert team of experienced prodromal and exercise investigators will follow a group of 15 UHR adolescent and young adults (ages 16-24) through a 12 week exercise trial to determine which level of exercise intensity/frequency is tolerable for participants and optimal for improving aerobic fitness (65% of VO2max and 2 sessions per week versus 85% intensity and 3 sessions per peek) and if improvements in aerobic fitness (i.e., VO2max, VO2peak, ventilatory threshold) are associated with increases in medial temporal structure volume (hippocampus and parahippocampal gyrus) and accompanying improvements in cognitive function (i.e., including tasks known to recruit heavily on medial temporal structures) as well as symptomatology and social/role functioning. If the benchmarks are met, this data will be used to streamline a three-year rater-blind controlled trial (15 UHR-exercise, 15 UHR waitlist-control) to determine the efficacy of the intervention in promoting medial temporal health as well as accompanying cognitive, clinical, and socio-occupational function improvement. Participants will be followed up to 24-months to determine if the intervention has an affect on clinical course and transition to psychosis. Taken together, this study is important for understanding the lessons necessary for planning a future large-scale trial, and has the potential to shed light on a promising new treatment for UHR youth.