Context: Evidence of an association between dietary patterns derived a posteriori and risk of cancer has not been reviewed comprehensively. Objective: The aim of this review was to investigate the relation between a posteriori-derived dietary patterns, grouped as healthy or unhealthy, and cancer risk. The relation between cancer risk and background characteristics associated with adherence to dietary patterns was also examined. Data Sources: PubMed and Embase electronic databases were searched. Study Selection: A total of 93 studies including over 85 000 cases, 100 000 controls, and 2 000 000 exposed individuals were selected. Data Extraction: Data were extracted from each identified study using a standardized form by two independent authors. Results: The most convincing evidence (significant results from prospective cohort studies) supported an association between healthy dietary patterns and decreased risk of colon and breast cancer, especially in postmenopausal, hormone receptor-negative women, and an association between unhealthy dietary patterns and increased risk of colon cancer. Limited evidence of a relation between an unhealthy dietary pattern and risk of upper aerodigestive tract, pancreatic, ovarian, endometrial, and prostatic cancers relied only on case-control studies. Unhealthy dietary patterns were associated with higher body mass index and energy intake, while healthy patterns were associated with higher education, physical activity, and less smoking. Potential differences across geographical regions require further evaluation. Conclusions: The results suggest a potential role of diet in certain cancers, but the evidence is not conclusive and may be driven or mediated by lifestyle factors.