Paraneoplastic neurologic syndromes (PNS) are a group of disorders that affect the central and the peripheral nervous system and frequently occur in patients with cancer which usually still is undiagnosed by the time the patient presents the first neurological manifestations. The discovery in the serum and cerebrospinal fluid of PNS patients of antibodies that target tumor antigens that also are normally expressed in the nervous system had a significant impact. First, the research on neuronal antibodies confirmed that most PNS are autoimmune disorders triggered by the underlying cancer supporting the use of immunotherapy to treat them; second, although the first antibodies described recognized intracellular neuronal antigens and therefore they were not pathogenic, these antibodies became robust biomarkers for the strict diagnosis of PNS; and third, the methodological approach used to characterize the first neuronal antibodies paved the way to the identification of antibodies against neuronal surface antigens that are pathogenic and responsible for some PNS and non-paraneoplastic encephalitis. Future studies should address several issues: (1) to improve the efficiency of commercial kits; (2) to provide strict criteria to select which neural antibodies should be used for the diagnosis of PNS; and (3) define in more detail the autoimmune mechanisms responsible for the brain injury in the PNS.