The restless legs syndrome (RLS) consists of four obligatory features: (1) an urge to move the legs usually accompanied or caused by uncomfortable and unpleasant sensations in the legs; (2) the urge to move, or unpleasant sensations begin or worsen during periods of rest or inactivity such as lying or sitting; (3) the urge to move, or unpleasant sensations are partially or totally relieved by movement such as walking or stretching, at least as long as the activity continues; and (4) the urge to move, or unpleasant sensations are worse in the evening or night than during the day or only occur in the evening or night. The cause of idiopathic and familial RLS is unknown, although the most important pathophysiologic clues come from autopsy material showing that iron is decreased in the substantia nigra of patients with RLS. There is also some evidence that underactivity of the endogenous dopaminergic and the endogenous opioid system with its enkephalins and endorphins may be pathogenetic as well. Secondary forms of RLS may be associated with peripheral neuropathy and radiculopathy, low ferritin levels, renal failure, rheumatoid arthritis, fibromyalgia, and multiple sclerosis. Pregnancy can be an exacerbating factor for RLS. Dopaminergic agonists are the first line of treatment, but opioids, gabapentin (Neurontin), and benzodiazepines are effective as well. Periodic limb movements in sleep (PLMS) are present in many but not all adult patients with RLS (80%), and PLMS also occurs with high prevalence in normal older adults even if RLS is not present. PLMS is therefore neither a sensitive nor specific-enough index of RLS to be diagnostic of RLS. PLMS without RLS rarely results in symptomatic sleep disruption, but in cases where symptomatic sleep disruption in the absence of RLS can definitely be attributed to the PLMS, the diagnosis of periodic limb movement disorder is made. However, it is not uncommon for a patient's PLMS to disrupt the sleep of the bed partner.