The Restless Legs Syndrome (RLS) is a sensorimotor disorder that has only recently been extensively investigated by validated methods. Following the first presentation of diagnostic criteria by the International RLS Study Group in 1995, several methods were specifically developed for clinical trials in RLS or adapted from other areas of medicine and health sciences. We present a critical overview on validated methods to assess (1) severity of RLS symptoms [International RLS Rating Scale (IRLS), John Hopkins RLS Severity Scale (JHRLSSS), the RLS-6 scales, and the investigator-based Clincial Global Impressions (CGI)]; (2) quality of life [RLS Quality of Life Instrument (RLS-QLI), Hopkins RLS Quality of Life Questionnaire (RLSQoL), and the RLS Quality of Life Questionnaire (QoL-RLS)]; (3) sleep disturbances and sudden onset of sleep; (4) sleep laboratory methods (polysomnography, limb activity monitoring by actigraphy) to evaluate sleep and periodic leg movements (including the "suggested immobilization test"); and (5) severity of augmentation (Augmentation Severity Rating Scale, ASRS). It is concluded that several validated methods are available to investigate the main features of RLS in practice and in clinical trials; however, further developments are needed to address new questions like the consequences of RLS on life functioning in areas such as ability to travel, days missed at work or impaired work performance.