Neurodegenerative diseases are resulting from a progressive loss of specific neuronal populations. The selective involvement of a given neuronal subpopulation ultimately determines the clinical phenotype of the disease. In most cases, these disorders are age-associated, typically manifesting in adulthood, with incidence increasing over age. The underlying pathophysiology involves abnormal deposits of otherwise correctly synthesized and functional proteins—such as beta amyloid, tau protein, alpha-synuclein, or TDP-43 protein—that become toxic to neurons, triggering a cascade of subsequent pathological events culminating in premature neuronal cell death. The classification system of neurodegenerative diseases is thus based on the pathophysiological background and offers both clarity and logical coherence, particularly given that the definitive diagnosis of neurodegeneration is always neuropathological. Recent findings suggest that different neurodegenerative diseases frequently coexist, leading to the emergence of three distinct terminologies to describe these overlapping conditions: concomitant neurodegenerative diseases; concomitant neurod egenerative pathology; and mixed neuropathology / mixed dementia.