This article reviews the recent advances in our understanding of methylphenidate and its use in children with attention-deficit hyperactivity disorder. It begins by reviewing the diagnostic issues important in deciding who should be treated with methylphenidate. The two main issues concern the current criteria for diagnosing attention-deficit hyperactivity disorder and the impact of comorbid conditions. This is followed by a review of recent literature about the effects of methylphenidate. The primary focus is on some of the effects of the drug on cognition. Lastly, the side effects are reviewed.