Periodic leg movements in sleep (PLMS) are a frequent finding in polysomnography. The prevalence of PLMS is estimated to be 4-11% in adults. In childhood, PLMS rarely occur although medical conditions like sleep apnea syndrome or neuropsychiatric disorders can lead to high rates of PLMS. In the elderly, PLMS are also common in subjects without sleep disturbances. In sleep studies, PLMS are found most frequently in restless legs syndrome (RLS) and often occur in narcolepsy, sleep apnea syndrome and REM sleep behavior disorder. Some patients with otherwise unexplained insomnia or hypersomnia reveal an elevated number of PLMS, a condition defined as periodic limb movement disorder (PLMD). PLMS were found also in various medical and neurological disorders that do not primarily affect sleep. A summary of these is presented. In sleep disorders related to dopaminergic dysfunction such as RLS, PLMS are considered to be a symptom of the disease. In other disorders like primary insomnia, the clinical relevance of PLMS is still being controversially discussed. Studies with findings both pro and contra are referred. To date, only a few studies have evaluated the efficacy of therapeutic substances in reducing PLMS in PLMD patients. Their results need to be confirmed in controlled randomized trials.