Senile plaques and neurofibrillary tan­gles diagnostically define Alzheimer’s disease. Together with dementia they constitute the orig­inal description of this disease, which was formed almost a century ago. But the role of these lesions in the etiopathogenesis of Alzheimer’s disease is still not clear. Senile plaques are comprised of fibrillary deposits of amyloid β which are present in the center of the lesion, and surrounded by dystrophic neurites. Immunohistochemical studies have shown that senile plaques besides amyloid β and β-precursor protein also contain various enzymes and components of the extracellular matrix. The source of amyloid β in the senile plaques and the possible primary structure on which amyloid is deposited are still a matter of controversy. Neurofibrillary tangles are composed of a dif­ferent type of abnormal filamets, i.e. paired heli­cal filaments and straight filaments. These fila­ments appear as a result of the disruption of the normal neuronal cytoskeleton. Neurofibrillary tangles contain protein τ as their main anti­genic determinant, but besides that some other constituents, as ubiquitin, neurofilaments, amy­loid β / β-precursor protein and various enzymes are also present. The cause and mechanism of incorporation of these components into neu­rofibrillary tangles is still not resolved. This paper presents some new information about the composition of senile plaques and neu­rofibrillary tangles. The results of some recent studies suggest active participation and inter­action of these lesions in the pathogenesis of Alzheimer’s disease.