Undeclared allergen(s) in commercial food products are responsible for many food recalls, as reported by regulatory agencies in various countries, including the United States. Correct allergen labeling practices are essential for the safety of food-allergic consumers. However, this practice may be hindered by the introduction of allergens all along the food supply chain, including unintentionally through cross-contact. To understand the pervasiveness of undeclared allergen(s) in commercial food products, the objective of this review is to summarize the prevalence of undeclared milk, egg, hazelnut, peanut, soy, and gluten as detected by ELISA from previously published surveys. The prevalence of undeclared allergen(s) in products with or without an advisory statement was also summarized and compared. As compiled by this review, there are some food categories that may be at higher risk for containing undeclared allergen(s). However, the data on prevalence and amount of allergen present may vary widely within any particular allergen or food category. Factors, such as food survey product selection, geography, awareness of allergen/gluten issues, and/or the choice of ELISA method, may be responsible for such differences.