In this chapter we review several CNS disorders of probable autoimmune origin or of unclear aetiology that sometimes are considered in the differential diagnosis of autoimmune encephalitis. These syndromes include the CNS complications of systemic autoimmune disorders: IgG4-related disease, Behçet disease, systemic lupus erythematosus (SLE), and sarcoidosis. In each of them, neurological symptoms may precede the onset of systemic symptoms. Other ‘frontier disorders’ include several diseases associated with primary involvement of the vascular endothelium or blood vessels: cerebral amyloid angiopathy-related inflammation, Susac’s syndrome, and primary angiitis of the CNS, which can all present with isolated neurological manifestations. These syndromes are immune-mediated, do not present specific or pathogenic neuronal antibodies, and their diagnosis is based on well-established clinical criteria that sometimes include neuroimaging and histopathological features. The clinical presentation of these syndromes may mimic that of several autoimmune encephalitis: SLE can present with psychosis (thus, it may need the differential diagnosis with anti-NMDAR encephalitis); IgG4-related disease can present with meningoencephalitis; and Behçet disease can present with brainstem dysfunction and neuroimaging findings resembling those of chronic lymphocytic inflammation with pontine perivascular enhancement responsive to steroids (CLIPPERS). The clinical presentation of primary angiitis of the CNS and Susac syndrome can be indistinguishable from that of autoimmune encephalitis.