Dementia with Lewy bodies (DLB) is a primary degenerative dementia sharing features of Parkinson’s disease (PD) and most oen also of Alzheimer’s disease (AD). e diagnosis of DLB implies an underlying pathology, the Lewy body (LB), and all patients with DLB have these inclusions. LBs are intraneuronal insoluble brillar aggregations of the synaptic protein α-synuclein (Figure 62.1). Causal mutations in the gene encoding α-synuclein associate with familial PD (Polymeropoulos et al., 1997), the other major LB disease. LBs were originally described by Friedrich Lewy over a century ago (Lewy, 1913) but the major constituent protein was only identied as α-synuclein in 1997 (Spillantini et al., 1997). Lewy described these neuronal inclusions as intracytoplasmic, spherical or elongated, eosinophilic masses possessing a dense core and a peripheral halo, with neurones able to contain multiple inclusions (Figure 62.1). Patients with a primary degenerative dementia and these inclusions have DLB. As described in the preceding chapter, they oen also have clinical features that dierentiate them from other dementia syndromes. While the denition of DLB in itself seems simple, very few patients with DLB have only LB pathology, and the variety of other pathologies, neurochemical abnormalities and patterns of neuronal loss continue to be assessed and dened, with multiple phenotypes now acknowledged.