This study will look at the effectiveness of Methylphenidate Transdermal System (MTS) in treating adult ADHD. MTS has received FDA approval for childhood ADHD but this is the first trial for adult ADHD. Subjects will experience two screening visits and one baseline visit. Those who meet admission criteria will enter the double-blind phase. This will involve two 4-week treatment periods one of which will involve the use of MTS and the other a placebo patch. Subjects who complete the double-blind phase will be allowed to enter a 180-day, open-label MTS phase designed to assess long-term effects.