Participants will be classified as either Alzheimer's disease(AD) group, mild cognitive impairment(MCI) group, elderly normal controls or young normal controls. Specific inclusion criteria for each group is described below.  Inclusion Criteria:  [Inclusion criteria: AD]  Age : 55 - 90 Clinical Dementia Rating (CDR)=0.5 or 1 Diagnostic and Statistical Manual-IV(DSM-IV) criteria for dementia National Institute of Aging and the Alzheimer's Association (NIA-AA) Probable AD dementia Study partner or caregiver to accompany patient to all scheduled visits Written informed consent  [Inclusion criteria: MCI (amnestic)]  Age : 55 - 90 Clinical Dementia Rating (CDR)=0.5 Concern regarding a change in cognition (obtained from the subject, from an informant who knows the subject, or from a skilled clinician observing the subject) Lower performance in any cognitive domain that is greater than would be expected for the subject's age and educational background Preservation of independence in functional abilities Study partner or caregiver to accompany subject to all scheduled visits Written informed consent  [Inclusion criteria: Elderly normal controls]  Age : 55 - 90 Clinical Dementia Rating (CDR)=0 Those with contactable Informant Written informed consent  [Inclusion criteria: Young normal controls]  Age : 20 - 54 Clinical Dementia Rating (CDR)=0 Written informed consent  Exclusion Criteria:  [Exclusion criteria: general]  Past history or presence of major psychiatric illness (e.g. schizophrenia, bipolar disorder, alcohol/substance abuse or dependence, delirium) Significant neurologic or medical condition that can influence the mental state Contraindications for MRI scan (e.g. pacemaker, claustrophobia) Illiteracy Significant visual or hearing difficulty Taking investigational drug In pregnancy or breast-feeding