More than 50 years ago, methylphenidate immediate-release (MPH-IR) was found to be effective in relieving symptoms of attention-deficit/hyperactivity disorder. Although the exact mechanism of the action is unknown, the efficacy of MPH is thought to be mediated by blocking the reuptake of norepinephrine and dopamine into the presynaptic neuron and increasing the release of these neurotransmitters into the extraneuronal space. Because of its short duration of effect, MPH-IR had to be dosed multiple times daily. In recent years, several methyphenidate extended-release (MPH-ER) formulations have been developed. Methylphenidate hydrochloride for extended-release oral suspension (MEROS or Quillivant(TM) XR) is the first long-acting MPH-ER oral suspension developed to treat attention-deficit/hyperactivity disorder. It offers advantages when compared with other MPH-ER preparations for patients who cannot or prefer not to swallow or manipulate capsules or wear a transdermal patch, while maintaining comparable efficacy and duration of effect. The pharmacokinetic, efficacy and safety profiles of this controlled substance are reviewed.