Delirium is a common postoperative complication in older adults associated with adverse events including functional decline, longer lengths of stay, and risk of institutionalization. The purpose of this article is to systematically review preoperative risk factors associated with delirium following noncardiac surgery. A medical literature search was conducted using several bibliographic databases (PubMed, CINAHL, Cochrane, PsychInfo), supplemented by a manual search of the references of retrieved articles. Studies were retained for review after meeting strict inclusion criteria that included only operative patients with incident postoperative delirium diagnosed prospectively using criteria derived from the Diagnostic and Statistical Manual of Mental Disorders Third or Fourth Edition. Quantitative analyses included significance testing, homogeneity testing, and effect-size pooling. Twenty-five articles were included for review. The incidence of delirium ranged from 5.1% to 52.2%, with greater rates after hip fracture and aortic surgeries. This review found two scales, a clinical prediction rule, and a delirium risk classification system that were validated in other operative settings. Individual risk factor analysis suggested that cognitive impairment, older age, functional impairment, sensory impairment, depression, preoperative psychotropic drug use, psychopathological symptoms, institutional residence, and greater comorbidity were associated with postoperative delirium. Of the risk factors examined, evidence was most robust for an association between delirium and cognitive impairment or psychotropic drug use, with moderate effect sizes for both. Missing data and measurement differences did not allow for inferences to be made about other risk factors. Effect-size pooling supports the concept that delirium is a heterogeneous disorder with multiple risk factors. More research is needed to better identify patients at risk for postoperative delirium and to develop preventive strategies.