ADHD affects from 3 to 5% of children, persists into adolescence in 40 to 70% of these children and continues into adulthood in at least 50% of affected adolescents. Methylphenidate was the first medication shown to be effective in treatment for adults with ADHD and continues to be widely used. While the extended release formulations represent an improvement over the immediate release versions, significant problems remain for many patients. In particular, most have been designed with the goal of providing medication only during school hours and a short time period after school. In adults, there is a frequent need for much more extended duration of treatment. MTS is a new form of methylphenidate that provides medication in a transdermal patch delivery system. It has a very even, slowly ascending pharmacokinetic profile. MTS's very stable slowly increasing blood level should overcome the problems noted above with a delivery system that is more convenient for many patients. It is currently approved for treatment of childhood ADHD, with effectiveness and safety profiles similar to other forms of methylphenidate. This study will be the first to evaluate the effectiveness and safety of MTS in adult ADHD.