The delivery of nebulized drugs is poorly controlled and the choice of the most appropriate delivery device is poorly understood, particularly because of off-license prescriptions and a lack of evidence-based medicine. Standardized in vitro methods for measuring nebulizer performance have been adopted in Europe, by the 2001 publication of a European Standard, prEN13544-1. These standardized methods were subsequently incorporated within the European Respiratory Society nebulizer guidelines, which will provide clinicians with useful information to improve nebulizer therapies. Standards for measuring nebulizer performance should be considered in North America and elsewhere. Careful consideration should be given to either adopting the methods embodied in the European Standard or developing the basis for developing that standard further through the International Standards Organization. Either way, confusion among clinicians would be reduced and nebulizer safety and aerosol delivery efficiency increased by standardizing in vitro methods of nebulizer performance assessment.