Freezing of Gait (FoG) is a class of symptoms that occur in Parkinson's patients. Also called motor blocks, FoG is characterized by a sudden inability to move the lower extremities which usually lasts less than 10 seconds. The exact pathophysiology of FoG is poorly understood, but treatment with levodopa appears to improve FoG observed in the off-state. As Parkinson's patients progress in severity, FoG in the on-state can increase in frequency and appears to be resistant to dopaminergic therapies. There is additional evidence that norepinephrine as well as dopaminergic systems may be involved in FoG.