In the area of non-inferiority (NI) tests there are several aspects whose solutions are not definitively clarified. To highlight some are determination of the margin of NI, the calculation of test sizes, power comparison and biocreep. Determination of NI margin is very important which is not a clear issue directly influenced by the acceptability of a new drug. For the selection of the NI margin for anti-infective trials, FDA and CPMP have provided some general guidance. In this article, we examine suggestions from such guidance, as well as, several known margin functions for NI to have anti-infective trials.