Paraneoplastic neurological syndromes are rare non-metastatic complications of cancer that have an immune-mediated aetiology. The central and peripheral nervous systems are considered to be immune-privileged sites, since the presence of the 'blood-brain/nerve barrier' means that antigens sequestered within the nervous system do not normally induce an immune response. Aberrant expression of a neuronal antigen by a tumour arising outside this barrier can lead to the breakdown of immune tolerance to the nervous system. However, in many cases the immune mechanisms that result in neurological dysfunction remain poorly defined. Furthermore, aberrant expression of neuronal antigens can be detected in many tumours that are not complicated by non-metastatic neurological syndromes. This review article examines current concepts in the immunopathogenesis of paraneoplastic neurological syndromes.