In this review article we summarise the clinical features, epidemiology and differential diagnosis of the Restless Legs Syndrome (RLS) and Periodic Limb Movements in Sleep (PLMS). RLS and PLMS belong to the most common causes which disturb sleep, and the RLS prevalence in the general population is estimated to range from 5 to 15%. The RLS is encountered in 10-20% of patients presenting with a problem of initiating and maintaining sleep, and in over 10% of patients with excessive daytime sleepiness. The RLS is characterised by unpleasant sensations felt deep in the legs, usually between the ankle and knee, but occasionally involving also the upper limbs or the trunk, and occurring at rest, especially at bedtime. The paresthesias are accompanied by motor restlessness and by an urge to move the limb, which results in a temporary relief of the symptoms. In virtually all of the sleeping RLS patients PLMS are observed. These are stereotyped, periodic jerking movements of a non-epileptic origin consisting in an extension of the big toe and dorsiflexion of the ankle with an occasional flexion of the knee and hip. PLMS are accompanied by usually short awakenings or transient arousals as demonstrated by sleep LEG resulting in sleep disruption. In establishing the diagnosis of RLS/PLMS idiopathic and secondary forms should be distinguished, and in particular, a number of other conditions (e.g. uremia or iron-deficiency) need to be ruled out. The clinical diagnosis of the RLS is based on the criteria of The International Restless Legs Syndrome Study Group, and the most useful laboratory support is gained from all-night polysomnography and high-resolution actography.