This concept paper concerns a revision of the guideline directed to the requirements for demonstration of therapeutic equivalence between two inhaled products. The guideline focuses on hybrid applications but may be applicable also for other applications that are based on demonstration of therapeutic equivalence compared to a reference product, such as line extensions and variations. The guideline was originally published in September 2000 and was revised between September 2007 and January 2009 (henceforth referred to as Revision 1).