Neurofilaments are the major structural proteins of the neuronal cytoskeleton and are classified according to molecular weight into heavy, intermediate, and light chains. They are released into the interstitial fluid and cerebrospinal fluid (CSF) as a consequence of axonal damage. In particular, the light chain (NfL) represents the most abundant and soluble subunit and has been demonstrated to be increased in the CSF of patients with inflammatory, degenerative, vascular, or traumatic injuries in correlation with clinical and radiological activity. Similar results have been obtained measuring serum NfL with high-sensitivity single-molecule array, which enables reliable and repeatable measurement of the low NfL concentrations in serum. In particular, CSF and serum NfL values are strongly correlated in patients with multiple sclerosis (MS) and have been demonstrated to be increased in patients with MS and clinically isolated syndromes (CIS) in accordance with clinical and radiological activity. NfL levels increase in patients with a recent relapse and seem to predict cognitive impairment, long-term outcome, and conversion of CIS to MS. The few available data on patients with other demyelinating diseases suggest that NfL levels are also increased in neuromyelitis optica spectrum disorders and related conditions in correlation with attack severity, suggesting that axonal damage may occur in these disorders. We herein report and discuss published data on the role of NfL as a possible predictor of disease activity, clinical outcome and treatment response in patients with demyelinating conditions of the central nervous system.