Allergen extracts derived from natural sources contain the allergenic proteins necessary for allergen testing and immunotherapy. The production of and standardization of allergen extracts to ensure stability, potency, purity, consistency, and sterility is regulated by the US Food and Drug Administration. This chapter describes the regulatory framework, the manufacturing controls, and methods by which lot-to-lot consistency and potency are measured. A “standardized” extract implies a much more rigorous process to evaluate potency, linking the extract to true in vivo reactivity, while “non-standardized” extracts do not have that requirement. Thus, non-standardized extracts may potentially vary more widely in their content of allergenic proteins and in vivo reactivity. Controls at every level of production have been an ongoing process for many years and, as a result, the quality of allergen extracts available in the United States is continually improving.