In the development of formulations for pulmonary delivery and in the characterisation of different nebulisers, the particle size distribution (PSD) of aerosol exiting a nebuliser as well as the delivered dose are of significant importance.The European Standard for respiratory therapy equipment suggests two separate methods for aerosol characterisation: the multistage cascade impactor and laser diffraction for particle sizing and an additional method for testing the aerosol output rate.Optical particle counters (OPC) are an interesting alternative to cascade impaction and laser diffraction, due to their accuracy and resolution in obtaining PSDs.