Alzheimer’s disease typically manifests age 65 or older with a predominant memory dysfunction followed by a progressive impairment of other cognitive domains. Aging is the main risk factor for AD development. However, up to 10% of patients present an early onset (under 65), manifesting more frequently with atypical phenotypes. Amyloid plaques and neurofibrillary tangles due to tau deposition are the main hallmarks of the disease. Despite sharing the same neuropathological features, AD phenotypes present differential tau distribution patterns in cortical areas, being tau-pathology topographically related to the clinical syndrome. In addition to aging, several other factors may contribute to AD pathology and its clinical expression. AD is currently understood as a disease continuum starting with a preclinical phase, progressively leading to mild cognitive impairment and dementia. The development of biological and neuroimaging biomarkers detecting in vivo the defining features of AD has remarkably improved the accuracy and early diagnosis of AD in the last decades.