Various animal feed products may influence animal health, conversion of animal feed into food of animal origin and the emissions caused by animals. All these matters are regulated in the directives of the European Food Safety Authority (EFSA). This chapter first discusses EFSA guidance on how to compile dossiers for feed additives. The chapter then discusses key issues and steps in demonstrating the efficacy of new animal feed products: reduction of nitrogen (N) excretion, reduction of feed contamination of by mycotoxins and, finally, reduction of methane emissions with feed additives. Key steps, such as the use of in vitro and in vivo studies, to test the effects of feed additives are discussed in detail.