The proposed study will consist of six visits to the laboratory for a total of twenty-four hours, as follows: there will be an initial clinical screening visit (4 hrs), three repeated measures test days (6hrs each; two at baseline and one at 60 days), a mid-point visit (1hr), and a Lunch Box visit (10 minutes to obtain out of laboratory food). School-aged participants will not have to miss school to participate; if necessary, testing will be able to be performed over the course of the weekend. After the initial laboratory visit and informed consent (i.e. clinical screening visit), participants will be randomized at the Baseline 1 visit to one of two treatment groups to either receive placebo or MPH two times daily for a 2 month intervention period that starts at the Baseline 2 visit. In order to track physical activity energy expenditure, participants will be instructed to wear accelerometers for 1 week between the in lab screening visit and the Baseline 1 visit, and then again while under MPH or placebo for 1 week between the mid-point visit and the Final repeated measures test day. Upon leaving the Baseline 1 test day (i.e. no drug or placebo) participants will receive a 3 day supply of food from which they will exclusively eat from until they return to the lab 4 days later for the Baseline 2 repeated measures test day (i.e. under drug or placebo). Similarly, 4 days prior to the Final repeated measures test day, participants will again have to visit the laboratory for their "Lunch Box" visit to obtain their three days' supply of food from which they will eat exclusively from. Participants will return to the laboratory to complete the study at the Final (day 60) repeated measures test day after 3 days of eating from the "Lunch Boxes". For the 1 hour mid-point visit (at day 30) participants will be weighed and will report any perceived side-effects from the MPH or placebo. At this mid-point visit participants will bring in their pill container to verify compliance and to receive final 30 day supply of drug or placebo. At this time participants will be asked if they have begun any new medication or have started any over-the-counter medication as part of the study's safety monitoring protocol. A side-effects check list will also be provided to the participants upon leaving the Baseline 2 testing session; participants will be informed to note any perceived side-effects nightly for the first 14 days being on the study medication and will be instructed to notify the study coordinator as soon as specific side-effects are noted as being Major.