Sympathetic skin response (SSR), defined as the momentary change of the electrical potential of the skin, may be spontaneous or reflexively evoked by a variety of internal or by externally applied arousal stimuli. Although the suprasegmental structures influencing the SSR in humans are not well known, SSR has been proposed as a non-invasive approach to investigate the function of the sympathetic system. SSR is easy to apply but current procedures are not sufficiently reliable for diagnostic purposes, and show imperfect correlations both with clinical features and other measurements of autonomic, in particular, sudomotor dysfunction.