Background: The goal was to forecast the global burden of Alzheimer’s disease and evaluate the potential impact of interventions that delay disease onset or progression. Methods: A stochastic multi-state model was used in conjunction with U.N. worldwide population forecasts and data from epidemiological studies on risks of Alzheimer’s disease. Findings: In 2006 the worldwide prevalence of Alzheimer’s disease was 26.6 million. By 2050, prevalence will quadruple by which time 1 in 85 persons worldwide will be living with the disease. We estimate about 43% of prevalent cases need a high level of care equivalent to that of a nursing home. If interventions could delay both disease onset and progression by a modest 1 year, there would be nearly 9.2 million fewer cases of disease in 2050 with nearly all the decline attributable to decreases in persons needing high level of care. Interpretation: We face a looming global epidemic of Alzheimer’s disease as the world’s population ages. Modest advances in therapeutic and preventive strategies that lead to even small delays in Alzheimer’s onset and progression can significantly reduce the global burden of the disease.