Children's Growth and Behavior Study

* Eligibility Criteria for Parents/Guardians of Child Participants: INCLUSION CRITERIA: Parents/Guardians will qualify if they meet the following criteria. 1. Age \>=18 years 2. Have a child enrolled in this protocol (15-CH-0096). EXCLUSION CRITERIA: Parents/Guardians will be excluded for the following reasons: 1. If their child is not eligible to participate in the study (see below) 2. If they are believed by the medical study team to have a medical or psychiatric problem that will not allow them to complete study procedures safely (these will be determined on a case-by-case basis) Eligibility Criteria for Child Participants: INCLUSION CRITERIA: Volunteers will qualify if they meet the following criteria. * Age 8-17 years (NB: children may continue to participate as adults during follow-up). * Weight, height and BMI \>= 5th percentile for age and sex according to Centers for Disease Control and Prevention 2000 US standards. * Cognitively capable of completing study procedures (FSIQ \>= 70). * Good general health based on a normal history and physical examination (with the exception of overweight and minor, well-controlled illnesses). EXCLUSION CRITERIA: Individuals will be excluded (and provided treatment referrals as needed) for the following reasons: * History of major cardiovascular disease or any other serious obesity-related complication as assessed during history and physical exam. Individuals with untreated or major illnesses relating to the endocrine and/or cardiovascular systems are excluded because these illnesses will likely influence outcomes. Such obesity-related comorbidities include hypertension (defined by age- sex- and height- specific standards; and fasting hyperglycemia consistent with diabetes (fasting glucose \> 126 mg/dL). * Presence of other major illnesses: renal, hepatic, gastrointestinal, most endocrinologic (e.g., Cushing syndrome, untreated hyper- or hypothyroidism), hematological problems or pulmonary disorders (other than asthma not requiring continuous medication). Non-serious medical illnesses, such as seasonal allergies, will be reviewed on a case-by-case basis. * Regular use of any medication known to affect body weight or eating behavior (e.g., many medications prescribed for attention deficit hyperactivity disorder, or ADHD). Medication use for non-serious conditions (e.g., acne) will be considered on a case-by-case basis. * Current pregnancy or a history of pregnancy. A negative pregnancy test before starting the study will be required for postmenarcheal girls. * Current and regular use of tobacco products and/or alcohol. * A significant reduction in weight during the past three months, for any reason, exceeding 5% of body weight. * A history of significant or recent brain injury that may considerably influence performance on neurocognitive measures (i.e., any history of loss of consciousness \>=30 minutes associated with a head injury, any history of memory loss or hospitalization associated with a head injury, or \>=2 concussions within last year). * Presence of any significant, full-threshold psychiatric disorder based on DSM criteria such as schizophrenia, bipolar disorder, alcohol or substance abuse, anorexia or bulimia nervosa, or any disorder that, in the opinion of the investigators, would impede competence or compliance or possibly hinder completion of the study. These individuals will not be permitted to enroll in the current study and will be referred for treatment. Individuals who present with other psychiatric disorders, including sub-threshold psychiatric disorders, will be permitted to enroll in the study. Sub-threshold psychiatric disorders include symptoms that do not meet diagnostic threshold based on the DSM criteria for mental disorders, but which are nevertheless significantly impairing or distressing. If, based on the opinion of the investigators, a participant requires treatment for his/her psychiatric symptoms, the individual will be referred for treatment. Participants who develop any psychiatric disorder or significant psychiatric symptoms at any follow-up assessment during the study will not be excluded, but will be provided with treatment referrals. * Any other condition in the child or parents/guardians that, in the opinion of the investigators, would impede compliance or possibly hinder completion of the study (e.g., significant Learning Disorder). Additional exclusions for (optional) stool sample collection include: Stool Sample only: * Diagnosis or history of inflammatory bowel disease, including ulcerative colitis or Crohn's disease, celiac sprue, irritable bowel syndrome, or other inflammatory disorders of the intestine. * Diarrhea within 1 week prior to sampling. * Antibiotic use within 4 weeks prior to sampling. In addition, Experiments 1 and 2 have specific additional exclusions: Experiment 1 only: * Regular use of medications that could influence autonomic or endocrine functioning, including alpha and beta blockers, oral contraceptives, or prescription pain medication. * Scoring as "highly active" on the International Physical Activity Questionnaire (due to decreased cortisol reactivity). Experiment 2 only: * Participants will be excluded if MRI and MEG is contraindicated (metal in/on body, braces, presence of non-organic \[e.g., cochlear\] implants or cerebral clips, permanent tattooed makeup or general tattoos that are recent and/or in a dangerous location on the body or made with colors whose content in iron cannot be definitely ruled out. * Youth will be excluded if they are left-handed. All participants will receive a written explanation of the purposes, procedures, and potential hazards of the study. Communication of this information and of the participant's assent as well as the consent of the parent or guardian will be documented in the medical record and copies of all signed documents given to each family. All participants will be informed of their right to withdraw from the study.