Inclusion Criteria:

1. At the time of consent, are between the ages of 18-55, inclusive.
2. Meet DSM-IV criteria for ADHD as assessed by the Adult ADHD Clinician Diagnostic Scale (ACDS) v1.2.
3. Female participants of childbearing potential must test negative for pregnancy at the time of enrollment based on a urine pregnancy test and agree to use a reliable method of birth control during the study. Females of childbearing potential are defined as women not surgically sterilized and are between menarche and 2 years post-menopause.
4. Must have a satisfactory medical assessment with no clinically significant abnormalities as determined by medical history, physical exam, ECG, and clinical laboratory testing.
5. Must be able to swallow capsules.
6. Must be able to begin the daily dose of study medication in the morning.
7. Must be off previous amphetamine or methylphenidate treatment for 1 week prior to baseline (visit 2). Must be off past non-stimulant ADHD medication (i.e., atomoxetine) for 3 weeks prior to baseline (visit 2).
8. In the opinion of the investigator, the subject must understand and be able, willing and likely to fully comply with the study procedures and restrictions.
9. Must have given signed and dated informed consent in accordance with Good Clinical Practice (GCP) Guidelines.

Exclusion Criteria:

1. Lifetime or present history of bipolar disorder, schizophrenia or schizoaffective disorder.
2. Uncontrolled comorbid major depressive disorder, anxiety disorder or dysthymia. Participants with controlled depressive or anxiety disorders may participate if their medications have been stabilized for a minimum of four weeks and, in the opinion of the Principal Investigator, will not interfere with adherence, safety, or efficacy assessments.
3. Anyone who meets current DSM-IV-TR criteria for alcohol or any non-alcohol substance abuse or dependence disorder (excluding nicotine).
4. Have organic brain disease (such as dementia) or traumatic brain injury residua. Have a history of seizure disorder (other than febrile seizures) or participants who have taken (or are currently taking) anticonvulsants for seizure control.
5. Females who are currently pregnant or breast feeding, and women of child-bearing potential who are not currently using an adequate form of birth control.
6. Participants with clinically significant ECG or laboratory abnormalities at screening that are deemed exclusionary in the opinion of the Principal Investigator.
7. Participants who work the night shift or another schedule that would preclude beginning the daily dose of study medication in the morning.
8. Participants with a positive urine drug result at Screening.
9. Participants with any concurrent chronic or acute illness or unstable medical condition that could, in the opinion of the study physician, confound the results of safety assessments, increase risk to the subject or lead to difficulty complying with the protocol. Subjects who have a history of mental retardation or severe learning disability will be excluded.
10. Participants with a history of structural cardiac abnormalities as well as any other condition that may affect cardiac performance.
11. Participants with documented history of allergy, intolerance, or non-responsivity to methylphenidate or amphetamines. This includes a history of two or more failed stimulant treatment trials, as deemed by the Principal Investigator.
12. Participants who in the investigator's opinion meet any of the exclusionary criteria specified on the FDA label of either Adderall or Adderall XR.