The randomized controlled trial (RCT) is considered the best interventional design to assess issues related to treatment and prevention. The RCTs can have different designs including superiority, equivalence, or non-inferiority design. A superiority trial aims to detect the potential superiority of new therapy compared to an active comparator or a placebo, an equivalence trial tends to demonstrate that a new therapy is an equivalent (within margins) to its active comparator, and a non-inferiority trial (NIT) is going to show that the new therapy is not worse than the comparator, as a typical active drug. Increasingly, major trials are conducted to see if the efficacy of a new treatment is as good as a standard treatment. The new treatment usually has some other advantages (e.g., fewer side effects, ease of administration, lower cost), making it worthwhile to demonstrate non-inferiority in respect to efficacy. Thus, NIT is going to determine whether a new treatment is not worse than a reference treatment by more than an acceptable amount. Among the challenges of NITs compared with superiority, trials are the choices of the non-inferiority margin (NIM), the primary population for analysis, and the comparator treatment considering several choices for the comparator arm in an NIT. This article is going to review the current knowledge about NIM.