Alzheimer’s disease(AD), which affects 5~10% of people over 65 years of age and up to 47% of those aged 85 years or older, is the most common cause of primary progressive dementia, followed by cerebrovascular and other neurodegenerative diseases, including diffuse Lewy body disease and frontotemporal dementia. AD is characterized by a progressive, global, and irreversible deterioration of cognitive function, which usually begins with memory problems, followed by deficits in language, mathematical, and visuospatial skills, abstract thinking, and planning, as well as personality and behavioral changes. Blood flow and metabolic imaging provides objective, accurate, and noninvasive diagnostic and therapeutic information about patients with dementia of various causes. Recent development in radiochemistry made it possible to evaluate noninvasively the pathologic and biochemical changes in AD at molecular levels. This review article describes the neurochemical changes in AD and their evaluation with PET and SPECT, focusing on the cholinergic system that is involved in the cognitive deterioration seen in AD