Delirium is a common and serious problem for older hospitalized patients. This investigation proposes a multifactorial model of delirium etiology, involving a complex interrelationship of predisposing (vulnerability) factors and precipitating factors (acute insults). An overview of risk factors for delirium identified in 14 studies published since 1980 is provided. Although these studies identify key risk factors for delirium, they do not allow the examination of the interrelationship of predisposing and precipitating factors. Thus, we present two prospective cohort studies by our group which empirically examine: (1) predisposing (vulnerability) factors, (2) precipitating factors, and (3) the interrelationship of predisposing and precipitating factors. Understanding these risk factors is the key to developing appropriate preventive strategies and to target intermediate and high risk patients for intervention efforts.