Inclusion Criteria:

• Ages 10 to 17 years (inclusive).
• Receiving treatment with olanzapine, quetiapine or risperidone, aripiprazole or ziprasidone for the past 56 days or longer.
• Clinically stable on current treatment regimen (see Rationales below).
• Stable dose of current psychotropic co-medications for at least 30 days.
• BMI increase of > 7% within 3 months OR a > 0.5 BMI z-score increase within the past 24 months while taking olanzapine, quetiapine or risperidone, with maintenance of the threshold level of weight gain.
• Lifetime diagnosis of a schizophrenia spectrum disorder (schizophrenia, schizoaffective disorder, psychotic disorder NOS), a bipolar spectrum disorder (bipolar disorder, bipolar disorder NOS), certain mood disorders (mood disorder NOS, major depressive disorder with psychotic features), or certain disruptive/aggressive disorders (conduct disorder, intermittent explosive disorder, autism spectrum disorder with history of clinically significant levels of disruptive behaviors as defined below) using DSM-IV criteria determined by clinical interview and K-SADS-PL.
• Sexually active girls must agree to use an effective form of birth control or be abstinent.
• Principle caretaker is able to participate in study appointments as is clinically indicated.
• guardian and the child must agree (legally consent and assent) to participation.

Exclusion Criteria:

• Any medication that would significantly alter glucose, insulin or lipid levels. Prohibited medications will include, but are not limited to: insulin, steroids, topiramate, sibutramine, orlistat, metformin, amantadine, vitamin E (other than in standard multivitamins), antidiabetic drugs, HIV drugs.
• Major neurological disorder or medical illness that affects weight gain (e.g., unstable thyroid disease), requires a prohibited systemic medication or procedure (e.g., diabetes mellitus [insulin], chronic renal failure [steroids]) or that would prevent participation in physical activity in the healthy lifestyle program.
• Current active thyroid (TSH >18 microIU/ml), hepatic (2 LFTs >4x upper limits of normal), renal (serum Creatinine >1.4 mg/dL in females and serum Creatinine >1.5 mg/dL in males), cardiac, gastrointestinal, or adrenal disease.
• Fasting glucose > 125 mg/dL on two occasions indicating need for prompt treatment for diabetes.
• Child meets DSM-IV criteria for substance abuse or dependence disorder within the past month, not including tobacco abuse or dependence • Current treatment with more than one antipsychotic medication.
• Current treatment with more than 5 total psychotropic medications (i.e., 4 psychotropics plus SGA).
• Known hypersensitivity to metformin.
• Pregnant or breast feeding.
• Current or lifetime diagnosis of anorexia nervosa or bulimia nervosa.
• Significant risk for dangerousness to self or to others that makes participating inadvisable.
• Language issues that prevent child and/or parent from completing assessments or treatment.
• Ongoing or previously undisclosed child abuse requiring new department of social service intervention.