The standardization of in vitro bench-top assessment of nebulizer performance could prove to be extremely beneficial in the advancement of aerosol delivered by nebulizers. At the present time, there is considerable confusion as to how different nebulizer systems perform. Clinical in vivo tests are often compromised, as high patient variability requires large study numbers before differences become significant. In vitro tests should offer less variability and much lower costs, but the test criteria must be standardized and be clinically representative. Standard methods to assess nebulizer performance should therefore be designed to reflect the clinical situation as far as practicable. This would incorporate test methods involving: 1. simulated breathing patterns to estimate the amount of aerosol released and inhaled 2. use of solute tracers to measure aerosol release of salts or drugs themselves 3. use of aerosol sizing instruments such as low-flow cascade impactors that would reflect in vivo deposition. Standards must be written to accommodate the variety of existing devices currently on the market. Because standards take a significant time (typically 1-3 years) to prepare and are intended to operate for a further 10 or more years, they should also aim to accommodate possible advancements in device technology. The potential for such developments should not be overlooked in preparing a standard for in vitro testing of nebulization devices.