Alzheimer's disease (AD) and vascular dementia (VaD) are nowadays widely accepted as the most common dementia forms. This chapter focuses on the overlap of the two dementia groups, discussing risk factors such as hypertension, symptoms, pathologies and possible mechanisms of interaction. Dementia of Alzheimer type is a syndrome occurring late in life. The textbook clinical picture of AD includes a long preclinical phase and an insidious onset of dementia, while the classical description of cognitive decline due to vascular causes comprises a step-wise onset and course. Two issues concerning the diagnosis of pure AD or VaD are identifying the actual pathological burden and relating it to cognitive impairment. Attempting to sharply distinguish between AD and VaD may lead to an oversimplification of reality. Therapeutic interventions aimed at a single dementia etiology might have only a limited efficacy. To influence the future occurrence of dementia diseases, effective preventive measures are required.