This talk offers a general discussion of regulatory, study design and statistical issues associated with the use of non-inferiority studies when these are used to establish the effectiveness of a new animal drug. We will discuss critical issues in non-inferiority studies such as assay sensitivity, the constancy assumption, and choice of the non-inferiority margin. These issues are specially challenging when designing active control veterinary trials because there are typically few historical placebo controlled studies, with small sample sizes and subjectively evaluated outcomes. The implications of FDA current guidance such as ICH E9 and ICH E10, and the draft guidance "Non-Inferiority Clinical Trials" will be discussed from a statistical perspective.