Dementia is becoming increasingly prevalent since elderly patients are living longer due to the development of treatments for other diseases and conditions. The percent of our population over 60 is also increasing with the wave of aging baby boomers. Additionally, more individuals seek medical assistance for cognitive problems as visibility for treatments improves. This combination of factors results in the dementia syndromes becoming more common, causing physicians to encounter more patients with dementia as well as more caregivers of these patients. Of dementia subtypes, Alzheimer's disease (AD) is the most common. Dementia with Lewy bodies (DLB) is thought to be the second most common subtype. DLB's typical symptoms include cognitive impairment, visual hallucinations, spontaneous parkinsonism, and fluctuating confusion. Supportive features include a variety of sleep disruptions that may occur before manifestations of dementia. Psychiatric symptoms include vivid visual hallucinations and depression. The clinical features of DLB are strikingly similar to those of dementia in Parkinson's disease (PD). The underlying biology of DLB is complex, but the presence of alpha-synuclein containing Lewy bodies (LB) is a common factor. These inclusions also contain ubiquitin. PD dementia shares these pathological findings with DLB, as well as neural degeneration of the substantia nigra. DLB and dementia in PD may represent the same pathological process along a disease spectrum. Additionally, many DLB cases are also associated with beta-amyloid and tau-containing neurofibrillary tangles, features that are associated with AD. Frequently, AD patients are also found to have LB. The reason for this overlap is unknown. However, the greater the Alzheimer's pathology in DLB patients, the more the clinical features of DLB overlaps with AD. In this chapter, we will review DLB including clinical, pathological, and radiological features as well as biomarkers and treatments.