The hallmarks of peripheral nervous system (PNS) disorders and neuromuscular diseases are weakness, numbness, or pain. Acute exacerbation of a neuromuscular disorder can have a rapid transition from unlabored breathing to decompensation and hypoventilation because of muscular fatigue or with aspiration of oral secretions in patients with marginal ventilatory function. Strength testing is performed by assessing the power of several proximal and distal muscle groups and by noting patterns of weakness and asymmetry. Numbness indicates the need for sensory testing to determine the patient's ability to perceive sensory stimuli. Deep tendon reflex (DTR) loss that is asymmetrical or focal in the distribution of one or several nerve root(s) or specific nerve(s) suggests localized disease. This chapter discusses disorders involving nerve roots, mixed nerves, peripheral nerves, disorders of neuromuscular transmission, disorders involving muscle and muscle function and motor neuron disease.