Usually, it is the purpose of a clinical trial to demonstrate the superiority of a (new) treatment in comparison to another treatment with regard to a well-defined criterion of efficacy. However, other aspects rather than improved efficacy might be regarded as advantages of a new therapy, i.e. less or less severe adverse events, a more simple applicability, or a lower price. In this case, it may be sufficient to show a "comparable" efficacy (therapeutic equivalence). Unfortunately, equivalence studies can lead to severe problems of interpretation in case of insufficient methodological planning. In general, more detailed information must be available in advance compared to the common (superiority) trials. Very carefully designed trials are necessary to evaluate the therapeutic equivalence of treatments.