Alzheimer disease (AD) is a neurodegenerative disorder characterized by the presence of β-amyloid (Aβ)-containing plaques and tau-containing neurofibrillary tangles. AD typically presents with amnestic cognitive impairment but can also manifest as non-amnestic cognitive impairment. The severity of cognitive impairment varies, ranging from subjective decline to dementia. AD is the most common cause of dementia, though it can coexist with other neurodegenerative or cerebrovascular conditions. The epidemiology of AD is closely linked to that of all-cause dementia, with increasing prevalence and incidence with age. Key risk factors include age, genetic factors such as *APOE* ε4, and modifiable factors like metabolic conditions and lifestyle choices. Pathophysiology involves synaptic dysfunction, loss of synaptic homeostasis, and dysfunctional proteostasis, particularly in the endosomal/lysosomal system. Aβ and tau interact in complex ways, contributing to synaptic and neuronal loss. Diagnosis of AD relies on clinical assessment, biomarkers, and imaging techniques, with a focus on early detection and management to improve quality of life.Alzheimer disease (AD) is a neurodegenerative disorder characterized by the presence of β-amyloid (Aβ)-containing plaques and tau-containing neurofibrillary tangles. AD typically presents with amnestic cognitive impairment but can also manifest as non-amnestic cognitive impairment. The severity of cognitive impairment varies, ranging from subjective decline to dementia. AD is the most common cause of dementia, though it can coexist with other neurodegenerative or cerebrovascular conditions. The epidemiology of AD is closely linked to that of all-cause dementia, with increasing prevalence and incidence with age. Key risk factors include age, genetic factors such as *APOE* ε4, and modifiable factors like metabolic conditions and lifestyle choices. Pathophysiology involves synaptic dysfunction, loss of synaptic homeostasis, and dysfunctional proteostasis, particularly in the endosomal/lysosomal system. Aβ and tau interact in complex ways, contributing to synaptic and neuronal loss. Diagnosis of AD relies on clinical assessment, biomarkers, and imaging techniques, with a focus on early detection and management to improve quality of life.