Orally inhaled drugs are drug-device combination products, and have been widely used as the first option for treatment of respiratory diseases such as asthma and chronic obstructive pulmonary disease(COPD). The development of inhaled drug products is not only full of tech. challenges, but also has disputes and discrepancies between USA and European regulatory agents. This paper compares and analyzes the FDA and EMA guidelines for the development of generic inhaled drug products on in vitro equivalence study, pharmacokinetic(PK) and pharmacodynamic(PD) bioequivalence studies, and discusses the scientific rationale. The in vitro equivalence for a generic inhaled drug is the essential requirement. For in vivo studies, PK study shows sensitive and objective as an indicator of bioequivalence, however, disputes exist for PD study, which needs more studies. In China, the development of generic inhaled drugs should be based on CDE guidelines, together with investigations of drug properties, dosage forms and updated regulations, to scientifically design the in vitro and in vivo evaluation studies.